Ssh – mRemoteNG succeeds whereas PuTTY fails

I have encountered a few cases (e.g. OSGi API, port 2222) where SSH connection via PuTTY fails to authenticate at login, while same exact authentication user/password succeeds via mRemoteNG.

PuTTY's failure symptom is by displaying a PuTTY Fatal Error message box with the following message:

"Server unexpectedly closed network connection"

with no corresponding log entry whatsoever in the server's /var/log/secure.

Both succeed through port 22 into same server. Both use SSH 2.0.

Enabling All session output in PuTTY logging, only logs this:

=~=~=~=~=~=~=~=~=~=~=~= PuTTY log 2017.12.26 13:11:05 =~=~=~=~=~=~=~=~=~=~=~=
login as: root
Further authentication required
SSH server: Password authentication
Using keyboard-interactive authentication.
Password: 

When I enabled SSH packets and raw data, putty.log included much more information. To keep this post brief, I am omitting the raw data:

=~=~=~=~=~=~=~=~=~=~=~= PuTTY log 2017.12.26 13:15:10 =~=~=~=~=~=~=~=~=~=~=~=
Event Log: Writing new session log (SSH raw data mode) to file: putty.log
Event Log: Connecting to 192.168.1.2 port 2222
Event Log: We claim version: SSH-2.0-PuTTY_Release_0.69
Incoming raw data at 2017-12-26 13:15:10
  00000000  53 53 48 2d 32 2e 30 2d 53 53 48 44 2d 43 4f 52  SSH-2.0-SSHD-COR
  00000010  45 2d 30 2e 39 2e 30 0d 0a                       E-0.9.0..
Event Log: Server version: SSH-2.0-SSHD-CORE-0.9.0
Event Log: Using SSH protocol version 2
Outgoing packet #0x0, type 20 / 0x14 (SSH2_MSG_KEXINIT)
...
Outgoing raw data at 2017-12-26 13:15:10
...
Incoming raw data at 2017-12-26 13:15:10
...
Incoming packet #0x0, type 20 / 0x14 (SSH2_MSG_KEXINIT)
...
Event Log: Using Diffie-Hellman with standard group "group14"
Event Log: Doing Diffie-Hellman key exchange with hash SHA-1
Outgoing packet #0x1, type 30 / 0x1e (SSH2_MSG_KEXDH_INIT)
...
Outgoing raw data at 2017-12-26 13:15:10
...
Incoming raw data at 2017-12-26 13:15:10
...
Incoming packet #0x1, type 31 / 0x1f (SSH2_MSG_KEXDH_REPLY)
...
Event Log: Host key fingerprint is:
Event Log: ssh-dss 1024 e6:85:76:7f:cb:6e:b6:21:b7:47:d0:86:a6:3f:3c:b6
Outgoing packet #0x2, type 21 / 0x15 (SSH2_MSG_NEWKEYS)
Outgoing raw data at 2017-12-26 13:15:10
...
Event Log: Initialised AES-256 SDCTR client->server encryption
Event Log: Initialised HMAC-SHA1 client->server MAC algorithm
Outgoing raw data at 2017-12-26 13:15:10
Incoming raw data at 2017-12-26 13:15:10
...
Incoming packet #0x2, type 21 / 0x15 (SSH2_MSG_NEWKEYS)
Event Log: Initialised AES-256 SDCTR server->client encryption
Event Log: Initialised HMAC-SHA1 server->client MAC algorithm
Outgoing packet #0x3, type 5 / 0x05 (SSH2_MSG_SERVICE_REQUEST)
...
Outgoing raw data at 2017-12-26 13:15:10
...
Incoming raw data at 2017-12-26 13:15:10
...
Incoming packet #0x3, type 6 / 0x06 (SSH2_MSG_SERVICE_ACCEPT)
...
Outgoing packet #0x4, type 50 / 0x32 (SSH2_MSG_USERAUTH_REQUEST)
...
Outgoing raw data at 2017-12-26 13:15:12
...
Incoming raw data at 2017-12-26 13:15:12
...
Incoming packet #0x4, type 51 / 0x33 (SSH2_MSG_USERAUTH_FAILURE)
...
Event Log: Further authentication required
Outgoing packet #0x5, type 50 / 0x32 (SSH2_MSG_USERAUTH_REQUEST)
...
Outgoing raw data at 2017-12-26 13:15:12
...
Event Log: Attempting keyboard-interactive authentication
Incoming raw data at 2017-12-26 13:15:13
...
Incoming packet #0x5, type 60 / 0x3c (SSH2_MSG_USERAUTH_INFO_REQUEST)
...
Outgoing packet #0x6, type 61 / 0x3d (SSH2_MSG_USERAUTH_INFO_RESPONSE)
...
Outgoing packet #0x7, type 2 / 0x02 (SSH2_MSG_IGNORE)
...
Outgoing raw data at 2017-12-26 13:15:15
...
Incoming raw data at 2017-12-26 13:15:15
...
Incoming packet #0x6, type 52 / 0x34 (SSH2_MSG_USERAUTH_SUCCESS)
Event Log: Access granted
Event Log: Opening session as main channel
Outgoing packet #0x8, type 90 / 0x5a (SSH2_MSG_CHANNEL_OPEN)
...
Outgoing raw data at 2017-12-26 13:15:15
...
Incoming raw data at 2017-12-26 13:15:15
...
Incoming packet #0x7, type 91 / 0x5b (SSH2_MSG_CHANNEL_OPEN_CONFIRMATION)
...
Event Log: Opened main channel
Outgoing packet #0x9, type 98 / 0x62 (SSH2_MSG_CHANNEL_REQUEST)
...
Outgoing raw data at 2017-12-26 13:15:15
...
Outgoing packet #0xa, type 98 / 0x62 (SSH2_MSG_CHANNEL_REQUEST)
...
Outgoing raw data at 2017-12-26 13:15:15
...
Event Log: Server unexpectedly closed network connection

Any idea what could be the reason for that difference between two seemingly compatible SSH clients?

UPDATE:

After finding out that mRemoteNG uses a modified version of PuTTY called mPuTTYNG (which looks and feels exactly like PuTTY, but unlike PuTTY it succeeds in connecting to the remote host), I enabled log for it too and compared the difference:

It appears that the two use a significantly different set of ciphers… Could that be the reason?

OTOH, both use Diffie-Hellman with same exact group and hash:

Event Log: Using Diffie-Hellman with standard group "group14"
Event Log: Doing Diffie-Hellman key exchange with hash SHA-1