Martin, I have posted exactly same bug long time ago, but you decided to ignore it. :)
https://winscp.net/forum/viewtopic.php?t=11082
Can you send me an email, so I can send you back a debug version of WinSCP to track the problem? Please include link back to this topic in your email. Also note in this topic that you have sent the email. Thanks.
You will find my address (if you log in) in my forum profile.