Sonoma did change the way wallpaper (and screensavers) get integrated (preferences have moved in an unusual place/preferences format too) which may be the root cause of this (although I admit I never hit that precise bug). Are you using classic images or some of the built in videos ?
If you are willing to try something, I would suggest an image (of your solid black color) instead.
Because of how the new system work, I wouldn't be surprised the bug is limited to those solid color backgrounds (which are handled quite differently for historic reasons, and it could very well be why you get the error).
The new wallpaper system is a bit of a mess, and solid colors have always been treated weirdly in their preferences.
