I had the same problem and found a workaround on reddit.
You can select all the pictures in your slideshow folder and right-click then "make as background"
It seems to work as before
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Windows 11 background slideshow was working, but after moving the original folder to another location, I can no longer set the background slideshow. While in the personalize menu, when i select "slideshow", the folder selection menu will appear for a split second and then disappear and change the background option to solid color. I assume it's defaulting to an invalid location, detecting an error, and closing. Is there a way to reset the default location so I can get past this bug?
I had the same problem and found a workaround on reddit.
You can select all the pictures in your slideshow folder and right-click then "make as background"
It seems to work as before