@HeiPet and all others,
I had the same failure and could rescue by following:
I uninstalled 3.5.11 via the Windows Control Panel, then installed 3.5.11 again and then did the 3.5.12 update, and that time it was fine.
So if you encounter also that issue, uninstall Dorico completely, then get the full installer for 3.5.12 via the Steinberg Download Assistant and install that.