Please attach a full debug and session log file showing the problem (using the latest version of WinSCP).
To generate the log files, set
Session.SessionLogPath. Submit the logs with your post as an attachment. You may want to remove data you consider sensitive, 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 logs publicly, you can mark the attachment as private.