Thanks for the quick reply!
I'm afraid the game still doesn't work, though. It does start (and so it did before), but it freezes on a black screen.
I pointed to the DRM issue because, well, there are ways to crack a game's Steamworks dependency (e.g. with the Goldberg Emulator), and if I replace the steam_api64.dll library provided in your build with a cracked one, then the game starts and plays just fine. But of course I'd prefer to be able to play the game without having to hack it. :P
PS: I must say I'm testing and playing your game on Linux with Wine so it'd be interesting to hear the experience of native Windows users as well. However, I don't think it should make any difference.