It occurs frequently if the library is not updated and some tracks are missing from the disk while are still in the database. It occurred me even with a synched library but you have to wait some time for it to show up.
Activate autoplay and go in kiosk mode.
Then click next button several times changing tracks randomly. Wit some time and leave the music on...
The vistaAct skin's background disappears after a few "not found tracks".
The skin refresh option does not make the background appear again

It doesn't seem to happen on other skins...
Regards.