@dutchscientist: First, I assume you mean 6.3.5 and 6.3.4.

So if you downgrade to 6.3.4, does the problem go away?
https://sourceforge.net/projects/winscp/files/WinSCP/

I'm not seeing this issue after about 10–15 minutes with FTPS. No minimum time was provided. I don't really think its SFTP related but it could be as easily as anything else.

Unknown if WinSCP was minimized or left open. Sounds like it may have been minimized. Tested minimized to no issue in about 10-15 minutes or more.

The hang could simply be from blocking the UI thread somewhere. I have seen WinSCP's UI thread blocked such as during listing where one can say its seeing a hang as its unresponsive until finished.

I have just updated to version 3.6.5. It opens fine, makes the connections to my Linux virtual computers, but if you leave it logged in, it later will not restore the window and needs to be stopped via the Task Manager. No error message reported, just not stable.

Did not have that problem with earlier versions.

Version 3.6.5 (Upgraded over 3.6.4)
tested on Windows 10 and 11 (Pro), 3 computers, each same problem
SFTP connection or multiple connections
No log file created or error message, program just hangs.