I did:
* a clean install
* used the offline installer files
* used run as admin on the shortcut

I would expect the game to start up. What happens is that it is busy for a moment, than nothing happens, not even an error message. (I cannot get even to the main menu, actually even the intro videos or commercials don't show up)

I saw that multiple users encountered this error, but didn't see a solution until now.

Does anybody know what to do?

Thanks!

Solved with the help of gog's support team.

Had to install it into C:\GOG_GAMES\ instead of my usual D:\ drive, and it worked.
I am not sure why WotC needs to be installed on C:, when the base game worked perfectly find on D:, but at least it works.

Might be useful for other encountering this error.