Csdrhrt

I'm trying to connect via sftp to my Synology NAS 215j running sftp server.
The sshd works great for root user, but when I do sftp as root, I'm getting Connection closed just after typing correct passphrase for my ssh key.

Here is the /etc/ssh/sshd_config:

Ciphers blowfish-cbc,aes256-cbc,aes256-ctr,aes128-ctr,aes128-gcm@openssh.com,aes192-ctr,aes256-gcm@openssh.com,chacha20-poly1305@openssh.com,

KexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1

MACs hmac-sha1,hmac-sha2-256,hmac-sha2-512,hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com,umac-128@openssh.com



Protocol 2

SyslogFacility AUTHPRIV

LogLevel ERROR

LoginGraceTime 60



MaxStartups 2

MaxAuthTries 3

MaxSessions 3



RSAAuthentication yes

PubkeyAuthentication yes

AuthorizedKeysFile    %h/.ssh/authorized_keys

RhostsRSAAuthentication no

IgnoreRhosts yes

ChallengeResponseAuthentication no

UsePAM no

PasswordAuthentication no

PermitEmptyPasswords no

PermitRootLogin without-password

TCPKeepAlive yes

ClientAliveInterval 600

ClientAliveCountMax 0

Banner /etc/ssh/issue.net

AllowTcpForwarding no

X11Forwarding no

UsePrivilegeSeparation sandbox

AcceptEnv LANG LC_*

UseDNS no



ChrootDirectory none

Subsystem       sftp    internal-sftp -f DAEMON -u 000



Match User root

    AllowTcpForwarding yes

Here is the verbose output:

$ sftp -P 22221 -vvv root@nas



OpenSSH_7.1p1, OpenSSL 1.0.2e 3 Dec 2015

debug1: Reading configuration data /usr/local/etc/ssh/ssh_config

debug2: ssh_connect: needpriv 0

debug1: Connecting to nas [192.168.1.51] port 22221.

debug1: Connection established.

debug1: identity file /Users/drewshg312/.ssh/id_rsa type 1

debug1: key_load_public: No such file or directory

debug1: identity file /Users/drewshg312/.ssh/id_rsa-cert type -1

debug1: key_load_public: No such file or directory

debug1: identity file /Users/drewshg312/.ssh/id_dsa type -1

debug1: key_load_public: No such file or directory

debug1: identity file /Users/drewshg312/.ssh/id_dsa-cert type -1

debug1: key_load_public: No such file or directory

debug1: identity file /Users/drewshg312/.ssh/id_ecdsa type -1

debug1: key_load_public: No such file or directory

debug1: identity file /Users/drewshg312/.ssh/id_ecdsa-cert type -1

debug1: key_load_public: No such file or directory

debug1: identity file /Users/drewshg312/.ssh/id_ed25519 type -1

debug1: key_load_public: No such file or directory

debug1: identity file /Users/drewshg312/.ssh/id_ed25519-cert type -1

debug1: Enabling compatibility mode for protocol 2.0

debug1: Local version string SSH-2.0-OpenSSH_7.1

debug1: Remote protocol version 2.0, remote software version OpenSSH_6.6p2-hpn14v4

debug1: match: OpenSSH_6.6p2-hpn14v4 pat OpenSSH_6.5*,OpenSSH_6.6* compat 0x14000000

debug2: fd 5 setting O_NONBLOCK

debug1: Authenticating to nas:22221 as 'root'

debug2: compat_kex_proposal: original KEX proposal: curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1

debug2: Compat: skipping algorithm "curve25519-sha256@libssh.org"

debug2: compat_kex_proposal: compat KEX proposal: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1

debug3: put_host_port: [nas]:22221

debug3: hostkeys_foreach: reading file "/Users/drewshg312/.ssh/known_hosts"

debug3: record_hostkey: found key type ECDSA in file /Users/drewshg312/.ssh/known_hosts:10

debug3: load_hostkeys: loaded 1 keys from [nas]:22221

debug3: order_hostkeyalgs: prefer hostkeyalgs: ecdsa-sha2-nistp256-cert-v01@openssh.com,ecdsa-sha2-nistp384-cert-v01@openssh.com,ecdsa-sha2-nistp521-cert-v01@openssh.com,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521

debug1: SSH2_MSG_KEXINIT sent

debug1: SSH2_MSG_KEXINIT received

debug2: kex_parse_kexinit: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1

debug2: kex_parse_kexinit: ecdsa-sha2-nistp256-cert-v01@openssh.com,ecdsa-sha2-nistp384-cert-v01@openssh.com,ecdsa-sha2-nistp521-cert-v01@openssh.com,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-ed25519-cert-v01@openssh.com,ssh-rsa-cert-v01@openssh.com,ssh-ed25519,ssh-rsa

debug2: kex_parse_kexinit: chacha20-poly1305@openssh.com,aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm@openssh.com,aes256-gcm@openssh.com,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,rijndael-cbc@lysator.liu.se

debug2: kex_parse_kexinit: chacha20-poly1305@openssh.com,aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm@openssh.com,aes256-gcm@openssh.com,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,rijndael-cbc@lysator.liu.se

debug2: kex_parse_kexinit: umac-64-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-sha1-etm@openssh.com,umac-64@openssh.com,umac-128@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-sha1,hmac-md5-etm@openssh.com,hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com,hmac-md5-96-etm@openssh.com,hmac-md5,hmac-ripemd160,hmac-ripemd160@openssh.com,hmac-sha1-96,hmac-md5-96

debug2: kex_parse_kexinit: umac-64-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-sha1-etm@openssh.com,umac-64@openssh.com,umac-128@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-sha1,hmac-md5-etm@openssh.com,hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com,hmac-md5-96-etm@openssh.com,hmac-md5,hmac-ripemd160,hmac-ripemd160@openssh.com,hmac-sha1-96,hmac-md5-96

debug2: kex_parse_kexinit: none,zlib@openssh.com,zlib

debug2: kex_parse_kexinit: none,zlib@openssh.com,zlib

debug2: kex_parse_kexinit:

debug2: kex_parse_kexinit:

debug2: kex_parse_kexinit: first_kex_follows 0

debug2: kex_parse_kexinit: reserved 0

debug2: kex_parse_kexinit: curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1

debug2: kex_parse_kexinit: ssh-rsa,ssh-dss,ecdsa-sha2-nistp256

debug2: kex_parse_kexinit: blowfish-cbc,aes256-cbc,aes256-ctr,aes128-ctr,aes128-gcm@openssh.com,aes192-ctr,aes256-gcm@openssh.com,chacha20-poly1305@openssh.com,

debug2: kex_parse_kexinit: blowfish-cbc,aes256-cbc,aes256-ctr,aes128-ctr,aes128-gcm@openssh.com,aes192-ctr,aes256-gcm@openssh.com,chacha20-poly1305@openssh.com,

debug2: kex_parse_kexinit: hmac-sha1,hmac-sha2-256,hmac-sha2-512,hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com,umac-128@openssh.com

debug2: kex_parse_kexinit: hmac-sha1,hmac-sha2-256,hmac-sha2-512,hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com,umac-128@openssh.com

debug2: kex_parse_kexinit: none,zlib@openssh.com

debug2: kex_parse_kexinit: none,zlib@openssh.com

debug2: kex_parse_kexinit:

debug2: kex_parse_kexinit:

debug2: kex_parse_kexinit: first_kex_follows 0

debug2: kex_parse_kexinit: reserved 0

debug1: kex: server->client chacha20-poly1305@openssh.com <implicit> none

debug1: kex: client->server chacha20-poly1305@openssh.com <implicit> none

debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<8192<8192) sent

debug1: got SSH2_MSG_KEX_DH_GEX_GROUP

debug2: bits set: 1041/2048

debug1: SSH2_MSG_KEX_DH_GEX_INIT sent

debug1: got SSH2_MSG_KEX_DH_GEX_REPLY

debug1: Server host key: ecdsa-sha2-nistp256 SHA256:SkJiuE7k6Z2ooVXv2cb4PGTeXgh+xNjxMtDG+8Pfqw0

debug3: put_host_port: [192.168.1.51]:22221

debug3: put_host_port: [nas]:22221

debug3: hostkeys_foreach: reading file "/Users/drewshg312/.ssh/known_hosts"

debug3: record_hostkey: found key type ECDSA in file /Users/drewshg312/.ssh/known_hosts:10

debug3: load_hostkeys: loaded 1 keys from [nas]:22221

debug3: hostkeys_foreach: reading file "/Users/drewshg312/.ssh/known_hosts"

debug3: record_hostkey: found key type ECDSA in file /Users/drewshg312/.ssh/known_hosts:10

debug3: load_hostkeys: loaded 1 keys from [192.168.1.51]:22221

debug1: Host '[nas]:22221' is known and matches the ECDSA host key.

debug1: Found key in /Users/drewshg312/.ssh/known_hosts:10

debug2: bits set: 1039/2048

debug2: set_newkeys: mode 1

debug1: SSH2_MSG_NEWKEYS sent

debug1: expecting SSH2_MSG_NEWKEYS

debug2: set_newkeys: mode 0

debug1: SSH2_MSG_NEWKEYS received

debug1: Roaming not allowed by server

debug1: SSH2_MSG_SERVICE_REQUEST sent

debug2: service_accept: ssh-userauth

debug1: SSH2_MSG_SERVICE_ACCEPT received

debug1: pubkey_prepare: ssh_get_authentication_socket: Connection refused

debug2: key: /Users/drewshg312/.ssh/id_rsa (0x7f97fc000610),

debug2: key: /Users/drewshg312/.ssh/id_dsa (0x0),

debug2: key: /Users/drewshg312/.ssh/id_ecdsa (0x0),

debug2: key: /Users/drewshg312/.ssh/id_ed25519 (0x0),

debug3: input_userauth_banner

                             )        (       )     )

   (      *   )  *   )    ( /(   *   )) ) ( /(  ( /(

   )   ` )  /(` )  /((   )())` )  /(()/( )()) )())

((((_)(  ( )(_))( )(_)) ((_)  ( )(_))(_)|(_) ((_)

 ) _ )(_(_())(_(_()|(_) _((_)(_(_()|_))   ((_) _((_)

 (_)_(_)_   _||_   _| __| | ||_   _|_ _| / _ | | |

  / _    | |    | | | _|| .` |  | |  | | | (_) | .` |

 /_/ _  |_|    |_| |___|_|_|  |_| |___| ___/|_|_|



UNAUTHORIZED ACCESS TO THIS SYSTEM IS FORBIDDEN AND WILL BE

PROSECUTED BY LAW!



DISCONNECT IMMIDIATELY IF YOU ARE NOT AUTHORIZED PERSON!



By accessing this system you agree that all your actions

will be monitored and logged.

debug1: Authentications that can continue: publickey

debug3: start over, passed a different list publickey

debug3: preferred publickey,keyboard-interactive,password

debug3: authmethod_lookup publickey

debug3: remaining preferred: keyboard-interactive,password

debug3: authmethod_is_enabled publickey

debug1: Next authentication method: publickey

debug1: Offering RSA public key: /Users/drewshg312/.ssh/id_rsa

debug3: send_pubkey_test

debug2: we sent a publickey packet, wait for reply

debug1: Server accepts key: pkalg ssh-rsa blen 535

debug2: input_userauth_pk_ok: fp SHA256:NWHiMGnH1Sz8K/cLRV9x493V6B6P8+oat6xtwFViZl4

debug3: sign_and_send_pubkey: RSA SHA256:NWHiMGnH1Sz8K/cLRV9x493V6B6P8+oat6xtwFViZl4

Enter passphrase for key '/Users/drewshg312/.ssh/id_rsa':

debug1: Authentication succeeded (publickey).

Authenticated to nas ([192.168.1.51]:22221).

debug2: fd 6 setting O_NONBLOCK

debug3: fd 7 is O_NONBLOCK

debug1: channel 0: new [client-session]

debug3: ssh_session2_open: channel_new: 0

debug2: channel 0: send open

debug1: Requesting no-more-sessions@openssh.com

debug1: Entering interactive session.

debug2: callback start

debug2: fd 5 setting TCP_NODELAY

debug3: ssh_packet_set_tos: set IP_TOS 0x08

debug2: client_session2_setup: id 0

debug1: Sending subsystem: sftp

debug2: channel 0: request subsystem confirm 1

debug2: callback done

debug2: channel 0: open confirm rwindow 0 rmax 32768

debug2: channel 0: rcvd adjust 87380

debug2: channel_input_status_confirm: type 99 id 0

debug2: subsystem request accepted on channel 0

debug2: channel 0: rcvd eof

debug2: channel 0: output open -> drain

debug2: channel 0: obuf empty

debug2: channel 0: close_write

debug2: channel 0: output drain -> closed

debug1: client_input_channel_req: channel 0 rtype exit-status reply 0

debug1: client_input_channel_req: channel 0 rtype eow@openssh.com reply 0

debug2: channel 0: rcvd eow

debug2: channel 0: close_read

debug2: channel 0: input open -> closed

debug2: channel 0: rcvd close

debug3: channel 0: will not send data after close

debug2: channel 0: almost dead

debug2: channel 0: gc: notify user

debug2: channel 0: gc: user detached

debug2: channel 0: send close

debug2: channel 0: is dead

debug2: channel 0: garbage collecting

debug1: channel 0: free: client-session, nchannels 1

debug3: channel 0: status: The following connections are open:

  #0 client-session (t4 r0 i3/0 o3/0 fd -1/-1 cc -1)



debug1: fd 0 clearing O_NONBLOCK

debug3: fd 1 is not O_NONBLOCK

Transferred: sent 4144, received 3136 bytes, in 0.2 seconds

Bytes per second: sent 24522.5, received 18557.5

debug1: Exit status 255

Connection closed

Please help! I'm pretty sure there is something in my sshd configuration, that I don't completely understand… I need someone to point my mistake

Thank you

UPDATE:

Here is the log info, I'v got from grep -i sftp /var/log/*:

/var/log/synoinfo.conf.bad:sftpPort="22"

/var/log/synoservice.log:Dec  5 12:54:25 nas-server entry.cgi_SYNO.Core.User.Home[1].set[9165]: service_reload.c:33 synoservice: reload [sftp].

/var/log/synoservice.log:Dec  5 12:54:25 nas-server entry.cgi_SYNO.Core.User.Home[1].set[9165]: service_reload.c:40 synoservice: [sftp] is not enabled, skip reload action

/var/log/synoservice.log:Dec  5 12:54:25 nas-server entry.cgi_SYNO.Core.User.Home[1].set[9165]: service_reload.c:53 synoservice: finish reload [sftp].

/var/log/synoservice.log:Dec  5 21:58:25 nas-server entry.cgi_SYNO.Core.Upgrade[1].start[22749]: service_pause_by_reason.c:27 synoservice: pause [sftp] by reason [smallupdate] ...

/var/log/synoservice.log:Dec  5 21:58:25 nas-server entry.cgi_SYNO.Core.Upgrade[1].start[22749]: service_pause.c:39 synoservice: [sftp] pausing ...

/var/log/synoservice.log:Dec  5 21:58:25 nas-server entry.cgi_SYNO.Core.Upgrade[1].start[22749]: service_pause.c:48 synoservice: [sftp] is already disable/pause.

/var/log/synoservice.log:Dec  5 21:58:25 nas-server entry.cgi_SYNO.Core.Upgrade[1].start[22749]: service_pause.c:89 synoservice: [sftp] paused.

/var/log/synoservice.log:Dec 12 19:56:27 nas entry.cgi_SYNO.Core.User.Home[1].set[16689]: service_reload.c:33 synoservice: reload [sftp].

/var/log/synoservice.log:Dec 12 19:56:27 nas entry.cgi_SYNO.Core.User.Home[1].set[16689]: service_reload.c:40 synoservice: [sftp] is not enabled, skip reload action

/var/log/synoservice.log:Dec 12 19:56:27 nas entry.cgi_SYNO.Core.User.Home[1].set[16689]: service_reload.c:53 synoservice: finish reload [sftp].

/var/log/synoservice.log:Dec 14 14:09:02 nas entry.cgi_SYNO.Core.User.Home[1].set[27314]: service_reload.c:33 synoservice: reload [sftp].

/var/log/synoservice.log:Dec 14 14:09:02 nas entry.cgi_SYNO.Core.User.Home[1].set[27314]: service_reload.c:40 synoservice: [sftp] is not enabled, skip reload action

/var/log/synoservice.log:Dec 14 14:09:02 nas entry.cgi_SYNO.Core.User.Home[1].set[27314]: service_reload.c:53 synoservice: finish reload [sftp].

/var/log/synoservice.log:Dec 14 14:10:39 nas entry.cgi_SYNO.Core.User.Home[1].set[27623]: service_reload.c:33 synoservice: reload [sftp].

/var/log/synoservice.log:Dec 14 14:10:39 nas entry.cgi_SYNO.Core.User.Home[1].set[27623]: service_reload.c:40 synoservice: [sftp] is not enabled, skip reload action

/var/log/synoservice.log:Dec 14 14:10:39 nas entry.cgi_SYNO.Core.User.Home[1].set[27623]: service_reload.c:53 synoservice: finish reload [sftp].

/var/log/synoservice.log:Dec 15 18:00:53 nas entry.cgi_SYNO.Core.User.Home[1].set[13303]: service_reload.c:33 synoservice: reload [sftp].

/var/log/synoservice.log:Dec 15 18:00:53 nas entry.cgi_SYNO.Core.User.Home[1].set[13303]: service_reload.c:40 synoservice: [sftp] is not enabled, skip reload action

/var/log/synoservice.log:Dec 15 18:00:53 nas entry.cgi_SYNO.Core.User.Home[1].set[13303]: service_reload.c:53 synoservice: finish reload [sftp].

/var/log/synoservice.log:Dec 15 18:05:03 nas entry.cgi_SYNO.Core.User.Home[1].set[14018]: service_reload.c:33 synoservice: reload [sftp].

/var/log/synoservice.log:Dec 15 18:05:03 nas entry.cgi_SYNO.Core.User.Home[1].set[14018]: service_reload.c:40 synoservice: [sftp] is not enabled, skip reload action

/var/log/synoservice.log:Dec 15 18:05:03 nas entry.cgi_SYNO.Core.User.Home[1].set[14018]: service_reload.c:53 synoservice: finish reload [sftp].

/var/log/synoservice.log:Dec 17 19:42:01 nas entry.cgi_SYNO.Core.BandwidthControl.Protocol[1].set[12255]: service_reload.c:33 synoservice: reload [sftp].

/var/log/synoservice.log:Dec 17 19:42:01 nas entry.cgi_SYNO.Core.BandwidthControl.Protocol[1].set[12255]: service_reload.c:40 synoservice: [sftp] is not enabled, skip reload action

/var/log/synoservice.log:Dec 17 19:42:01 nas entry.cgi_SYNO.Core.BandwidthControl.Protocol[1].set[12255]: service_reload.c:53 synoservice: finish reload [sftp].

/var/log/synoservice.log:Dec 17 21:29:43 nas entry.cgi_SYNO.Core.FileServ.FTP.SFTP[1].set[17732]: service_start.c:108 synoservice: [sftp] starting ...

/var/log/synoservice.log:Dec 17 21:29:45 nas entry.cgi_SYNO.Core.FileServ.FTP.SFTP[1].set[17732]: service_start.c:206 synoservice: [sftp] started.

/var/log/synoservice.log:Dec 17 21:32:06 nas entry.cgi_SYNO.Core.FileServ.FTP.SFTP[1].set[18453]: service_pause.c:39 synoservice: [sftp] pausing ...

/var/log/synoservice.log:Dec 17 21:32:08 nas entry.cgi_SYNO.Core.FileServ.FTP.SFTP[1].set[18453]: service_pause.c:89 synoservice: [sftp] paused.

/var/log/synoservice.log:Dec 17 21:32:10 nas entry.cgi_SYNO.Core.FileServ.FTP.SFTP[1].set[18453]: service_start.c:108 synoservice: [sftp] starting ...

/var/log/synoservice.log:Dec 17 21:32:12 nas entry.cgi_SYNO.Core.FileServ.FTP.SFTP[1].set[18453]: service_start.c:206 synoservice: [sftp] started.

/var/log/synoservice.log:Dec 17 21:32:51 nas entry.cgi_SYNO.Core.FileServ.FTP.SFTP[1].set[18960]: service_pause.c:39 synoservice: [sftp] pausing ...

/var/log/synoservice.log:Dec 17 21:32:52 nas entry.cgi_SYNO.Core.FileServ.FTP.SFTP[1].set[18960]: service_pause.c:89 synoservice: [sftp] paused.

/var/log/synoservice.log:Dec 17 21:32:55 nas entry.cgi_SYNO.Core.FileServ.FTP.SFTP[1].set[18960]: service_start.c:108 synoservice: [sftp] starting ...

/var/log/synoservice.log:Dec 17 21:32:56 nas entry.cgi_SYNO.Core.FileServ.FTP.SFTP[1].set[18960]: service_start.c:206 synoservice: [sftp] started.

/var/log/synoservice.log:Dec 17 21:38:47 nas entry.cgi_SYNO.Core.FileServ.FTP.SFTP[1].set[19744]: service_pause.c:39 synoservice: [sftp] pausing ...

/var/log/synoservice.log:Dec 17 21:38:48 nas entry.cgi_SYNO.Core.FileServ.FTP.SFTP[1].set[19744]: service_pause.c:89 synoservice: [sftp] paused.

/var/log/synoservice.log:Dec 17 21:38:50 nas entry.cgi_SYNO.Core.FileServ.FTP.SFTP[1].set[19744]: service_start.c:108 synoservice: [sftp] starting ...

/var/log/synoservice.log:Dec 17 21:38:52 nas entry.cgi_SYNO.Core.FileServ.FTP.SFTP[1].set[19744]: service_start.c:206 synoservice: [sftp] started.

/var/log/synoservice.log:Dec 17 21:40:09 nas entry.cgi_SYNO.Core.FileServ.FTP.SFTP[1].set[20292]: service_pause.c:39 synoservice: [sftp] pausing ...

/var/log/synoservice.log:Dec 17 21:40:11 nas entry.cgi_SYNO.Core.FileServ.FTP.SFTP[1].set[20292]: service_pause.c:89 synoservice: [sftp] paused.

/var/log/synoservice.log:Dec 17 21:40:13 nas entry.cgi_SYNO.Core.FileServ.FTP.SFTP[1].set[20292]: service_start.c:108 synoservice: [sftp] starting ...

/var/log/synoservice.log:Dec 17 21:40:14 nas entry.cgi_SYNO.Core.FileServ.FTP.SFTP[1].set[20292]: service_start.c:206 synoservice: [sftp] started.

/var/log/synoservice.log:Dec 17 22:14:56 nas entry.cgi_SYNO.Core.FileServ.FTP.SFTP[1].set[22807]: service_pause.c:39 synoservice: [sftp] pausing ...

/var/log/synoservice.log:Dec 17 22:14:58 nas entry.cgi_SYNO.Core.FileServ.FTP.SFTP[1].set[22807]: service_pause.c:89 synoservice: [sftp] paused.

/var/log/synoservice.log:Dec 17 22:14:58 nas entry.cgi_SYNO.Core.FileServ.FTP.SFTP[1].set[22807]: service_stop.c:49 synoservice: [sftp] stopping ...

/var/log/synoservice.log:Dec 17 22:15:00 nas entry.cgi_SYNO.Core.FileServ.FTP.SFTP[1].set[22807]: service_stop.c:106 synoservice: [sftp] stopped.

/var/log/synoservice.log:Dec 17 22:15:02 nas entry.cgi_SYNO.Core.FileServ.FTP.SFTP[1].set[22990]: service_start.c:108 synoservice: [sftp] starting ...

/var/log/synoservice.log:Dec 17 22:15:03 nas entry.cgi_SYNO.Core.FileServ.FTP.SFTP[1].set[22990]: service_start.c:206 synoservice: [sftp] started.

It looks like there's a problem on the server side. As soon as you request the SFTP subsystem you're receiving an end-of-file and it's exiting with a 255.

debug2: subsystem request accepted on channel 0

debug2: channel 0: rcvd eof

...  

debug1: Exit status 255

Can you check out the logs on the server? Probably grep in /var/log/

grep -i sftp /var/log/* 

should return something helpful.