Try disabling those 3rd party VSTs to see if it’s one of them. If you do find it is connected to one then uninstall and reinstall just to rule out a temporary disk or data corruption before going to the software support desk. Might save you time.
I try. Same situation. I disable Uad and Overloud TH2…Cubase stops to work. I can only right click on a Cubase icon in Quick start and load a project, so cubase works. But If I right click on cubase icon and try to close windows Cubase doesn’t quit or close…