MisterT1968: Thx for the help.
Unfortunately, so far nothing really helped. There were no changes to the system since the last time the games worked. Interrestingly Lula is working virtually normally (with the exception of the random crashes).
Interestingly I have in the last days a large amount of "Service 'GalaxyVlientService' terminated unexpectedly. This happened x times." This started on the 25th January. Before this time I had no terminations at all. At this day M$ Update KB4023057 was installed. But even so. after this day I was able to play the games without an issue.
I cannot find any log files at all for either the games nor GoG.
Btw. I always start the games through GoG Galaxy App (version 2.0.57 Beta; the newer versions do not install/download), but also tried to start them from the game folder itself.
There was a bug in a earlier Galaxy version that would cause this bug. The service client would crash every time the connection was lost.
Sadly the updater itself is affected as well.
So in order to install the fix, you need to download the installer and install it. There is no need to uninstall the old version, you just install the new one over it.
However, do this right after Windows startup before you open Galaxy, because otherwise some files will stay locked until the next reboot.
After installing the new version, updates should work again.