I forgot to mention that the version I got from Stardock is reported as 1.7 already. I tried both patches anyway, prompting to override the already installed patch. Both lead to the SecuROM message that I don't have the right disc inserted (which I obviously don't own since it's a digital download).

The support replied that I'm the first one to report this problem... I'll update my graphics drivers as recommended by them but I doubt this is the reason, because the demo, which I tried in the meantime, works flawlessly. Imo Stardock f*cked up the digital download. I've already re-downloaded it as well (in case it was corrupted the first time) but I encountered the exact same behaviour. And I'm not too keen on re-downloading 3 GB again since that takes a few hours for me.