I have been running (Win 10 latest updates) different versions of Cubase (CB9, 10, 11, 12) for a long time without any issues.
Now I get an error message (see below) when starting CB 10.5 (as admin), BUT ONLY if I have run CB 12 prior to this. If I reboot I can start CB 10.5 without problem, but as soon as I run CB 12 and then CB10.5 again, it fails to start.