I had a similar problem after my PC updated to 1809 this morning. The desktop wasn't animating and new wallpapers wouldn't apply. The background previews for videos wouldn't display either. Uninstalling and reinstalling, then later purging had no effect.
What seems to have done the trick is updating the Nvidia display drivers. The backgrounds started previewing after this, but it took another reboot for the desktops to apply and animate properly.
I don't know if this is the same as the original problem, but it might help anyone searching the same symptoms.
