Aaden: Do you have v1.1 or the day-zero-patch currently installed?
Patch 1.2 will only work with a fresh installation and v1.1, but not with the day-zero-patch applied during game activation.
If you're using day-zero, refer to the opening post for a link to v1.1.
ooh ... never mind, I was 100% certain my GOG version was 1.1 already, since it downloaded and installed a patch right after installing the game ... now I see that I was mistaken - downloaded 1.1 from GOG and bam! immediately starts downloading 1.2.
Thanks for the help, slightly confusing with half the patching being automated and half requiring manual downloads :)