This problem just popped up for me less than a week ago. Before that, my slideshow was running without a hitch. No change in the folder or its location. It's impossible to click on the slideshow button, as it is only there for a nano second. I tried the shell approach, but as soon as I reboot, it reverts back to no slideshow. But, at least Windows Key + R and then:
shell:::{ED834ED6-4B5A-4bfe-8F11-A626DCB6A921} -Microsoft.Personalization\pageWallpaper, is a workaround.