Post upgrading UCSM firmware and/or installing internal Certificate Authority (CA) issued certificate on the UCSM, direct SSH via PuTTY client might get rejected with error “Remote side unexpectedly closed network connection”. This is due to the update of ciphers that happens with the firmware upgrade or certificate installation.
Solution is to either use OpenSSH client or the latest version of PuTTY client (0.78) (Ref 1, 2).
OpenSSH access format: ssh username@IPaddress
IT Findings 