Just an idea but this restriction could be related to the brand new User Interface and may this is why AC19 runs on this OS and AC20 does not.
What happens if you do what it says and switch to another Theme?
But I an not really surprised, Windows 2008 is a very old operating system and ARCHICAD usually supports only the last 2-3 versions of an OS:
Laszlo....do you know or can you point me to a place that shows the backward compatibility of Windows 10 and Archicad? I am currently in the slow jerky process of bringing old projects up to 18. Thanks