Сегодня вдруг при попытке подключится к серверу плагином FAR'а NetBox получил такое:
Server protocol violation: unexpected SSH2_MSG_UNIMPLEMENTED packet
Решение нашел тут.
For me the key was that on the server there was not implemented *"Diffie-Hellman group exchange"* key exchange algorithm.
(see Connection -> SSH -> KEX)
Moving this key exchange algorythm to the bottom of list and making going an algorithm *"Diffie-Hellman group 14"* first - that solved the problem for me.
Сделал так. и всё заработало.