Background suddenly gone black and cannot change

My windows7 pro desktop computer has suddenly (morning of January 6, 2020) displayed a black background. It cannot be changed via Personalization/Desktop Background. That setting shows the correct background, but only black is displayed. That setting although you can select a different background reverts only to the last background that was selected - and in any case, only black is displayed.
I followed the instructions in https://support.microsoft.com/en-us/...e-in-windows-7
but none of the points were valid on this installation, and all checks were ok.
Perhaps this is related to the recent drop of support for Windows 7?
I also checked recent updates and nothing was updated in the days before, except for windows defender. There were a few others today, but that was after the problem occurred, and they also do not seem suspicious.

No change with clean startup.

Is the problem limited to one User Account, or does it persist with other user accounts too?

Interesting, I enabled the Guest account - it had the standard windows background and I was able to change it!

I was able to fix it by selecting a theme (aero), and then reverting back to the old picture.