OK, please report the problem to PuTTY and ProFTPD (I assume it's the SFTP server you are using).
Trying to upgrade your ProFTPD server is also worth trying.
I have also tried WinSCP Version 5.16 beta (Build 9865) and received the same error. Only versions 5.15.5 and earlier are working.
I have also tried PuTTY versions 0.72 and 0.71 and received the same error.
I am unable to logon using PuTTY 0.73. I receive a similar error:
Remote side sent disconnect message type 11 (by application): "Unsupported Protocol Sequence"
Thanks for your report. Can you login with the latest version of PuTTY (0.73)?
Versions 5.15.5 and earlier work fine. I have sent log files from both versions.