Receiving "Unsupported protocol sequence" when using 5.16.2 RC

Versions 5.15.5 and earlier work fine. I have sent log files from both versions.

Thanks for your report. Can you login with the latest version of PuTTY (0.73)?

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"

I have also tried PuTTY versions 0.72 and 0.71 and received the same error.

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.

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.