Please attach a full session log file showing the problem (using the latest version of WinSCP).
To generate the session log file, set
Session.SessionLogPath. Submit the log with your post as an attachment. Note that passwords and passphrases not stored in the log. You may want to remove other data you consider sensitive though, such as host names, IP addresses, account names or file names (unless they are relevant to the problem). If you do not want to post the log publicly, you can mark the attachment as private.
Btw, you cannot get that error when connecting to FTP server. You might get it when connecting to SSH/SFTP server only. Make sure you are using the correct protocol.