Partial success!
I don’t know yet if the DLCs are installed correctly, but I tested the GOG version also without them, as I knew the base game works using the Steam version.
- Cloud sync (or uninstalling GOG Galaxy) did not make a difference.
- Removing the video did not make a difference.
Now, what worked:
- Windows Sandbox! The game runs really slow, with about 1 FPS, but I could enter the game world in the GOG version. I never thought this would come in handy, but it was the closest thing to a fresh system without a lot of effort.
- New user: Reinstalling & playing the game with a new Windows user also seems to work.
==> The issue is somehow related to my Windows user profile.
I’ll investigate further, and also try with DLCs, but for now, it seems playing is possible!
Thank you all. If I ever find the actual reason for the problems, I’ll report here.