I just installed Archicad 20 on the same computer I'm using Archicad 19. After installation, I tried to run it and got this error:
Sorry Archicad will not run on this computer
Problem: Incompatible Windows theme (Visual Styles are disabled).
ARCHICAD will not run if your Windows theme is Classic or a High Contrast theme. In order to run ARCHICAD, please open the Windows Control Panel
and choose a different Windows theme.
I've never had this issue before. The computer OS is Server 2008-R2.