Log file, please.
Before posting, please read how to report bug or request support effectively.
Bug reports without an attached log file are usually useless.
/log=C:\path\to\winscp.logcommand-line argument. 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.