I don't know why but I was able to at least start the game before but now I get this more often than not. I have applied the unofficial patch along with the reschanger from WSGF to make widescreen possible. When I actually get the game to start I've no glitches so I don't think it's the widescreen trick, more like the GOG version itself or it combined with the extra patch.

Anyone got an idea? I run Windows 64-bit which I believe could also be a reason for instability. I always run with admin. I haven't tried with compatibility mode however.