Desktop background black

The same happanend to me today. After Uninstall KB5120998 all returned to normal. 2 hours with chatGPT.

I have the same issue, right after installing (KB5120998) (26200.9278). It is now impossible to apply a backgound color using the section "personalization>background [SOLID COLOR] " and picking a color. If you pick a color it stays as long as you disconnect and reconnect again or reboot, and then is black again. Picking a local image is ok and does not go away after a disconnect or reboot, but the solid color is now impossible to apply permanently. 

The same thing happened to me a few months ago. Then I realized that the wallpaper I selected had been moved from its original location because I deleted it.

If you use an image as your desktop wallpaper and then delete that image from the desktop, the wallpaper may start behaving like this: it can turn black, switch back to the default background, or keep alternating between the two.

To fix this, save the image you want to use as wallpaper in a place where you won’t delete it. I personally move it to the C: drive first, and then set it as the wallpaper.

The background is being saved for the current session but not surviving a restart, so the recent update, theme synchronization, or a policy may be resetting personalization. Open Settings > Personalization > Background, choose Picture, select a local image, and confirm it remains after signing out and back in. Temporarily disable Windows theme synchronization under Accounts > Windows backup and test again. Verify the image still exists and is not in a temporary or disconnected OneDrive location. If this is a managed PC, run gpresult /h report.html and ask the administrator whether a desktop-wallpaper policy applies. Create a new local user and repeat the test; persistence there would isolate the original profile. Since the issue followed specific updates, review their known issues and submit the reproduction through Feedback Hub. Uninstall an update only after the profile and policy checks fail to explain the reset, and create a restore point first.