Well, during investigations, I found Windows 10 build 1903 showing as ready to install, so I installed that.
All is now well, so I guess it was some weird OS issue.
Thanks anyway Matt.

No ID 8194, but a lot of 12293. True, there is a lot of info about it, but not much of it is consistent across posts.
While investigating, there seems to be a more general error with VSS / System Protection. Cannot even open the System Protection page. Guess I need to sort that first...
Ah well, joys of Windows...