Disconnected: Server protocol violation: unexpected SSH2_MSG_UNIMPLEMENTED packet
If you've bumped into this error when connecting to a server with PuTTY then don't worry — it's easy to get fixed.
First — try to update PuTTY to the latest version. It's probably that you're using an older version. If it didn't help then do the following:
Open session configuration.
Connection -> SSH -> Kex
Find “Algorithm selection policy” list.
Find “Diffie-Hellman group exchange” line there.
Move this line down to the last position.
Done! Don't forget to save your session's configuration.