I got some help from tech support at Universal Audio, although eventually I just gave up, uninstalled the UA drivers, reinstalled the UA drivers, found that didn't help, repaired the UA drivers, found that didn't help, uninstalled the UA drivers, reinstalled the UA drivers, and finally I was able to load UA plugins inside of Cubase. Gack!
Cubase, on the other hand, remains ill. Although I can now open a project, when I do the Windows menu vanishes from the menu bar, although you can see the individual entries as you mouse over them, so it is not so much vanished as turned invisible. When you close a project and reopen it, the Mix Console window opens up and shows nothing in the area where you would normally see the plugins, routing, and faders other than the outlines for each of those windows. Then when you try to operate on it or close it, Cubase crashes.
Well, it worked for UA. Let's uninstall Cubase and reinstall it.
And guess what?
It didn't help.
I am becoming increasingly annoyed at this supposedly stable and safe-for-business update.
No, it appears that you can't revert to the earlier version of Windows 10 either...