nadenislamarre I did some more testing. For sure it has something to do with updating the system. I came from 5.25 and replaced the bootfiles as always (also replaced the bios files with newer ones - no missing bios files as before). I switched back to 5.25 the same way and all works fine again.
So I did a clean installation on a usb-drive with adding the gamefiles. A clean installation did not show this behaviour.