Ok, thanks for the reply. I actually found a "workaround" by going to the game's files and launching the game there directly. So at least it's playable somehow... But yeah, the game should be able to launch directly from Steam, because that is what anybody expects.
This happened with an earlier build. We think it has reverted to that build for some reason, or it broke again. We will do some more digging. We am glad you found the workaround. Thank you very much for finding this glitch. - GiG Dev team