It used to work just fine on my system with Proton 7.0-6 (or 7.0-5 ?) until the recent major steam client update. Since then the game doesn't start any more. No error is shown and the steam console output also does not provide any helpful errors.

I reinstalled steam and the game, tried all Proton versions mentioned here and I even forced steam to reinstall the old client as described here: https://steamcommunity.com/discussions/forum/0/6516193260168294059/ , but the game still does not start.

Now I need to dig out an old Windows machine to finish the game :-(