I started getting this error also, it was after I included jquery on the page above shadow box.
So I didn't bother trouble shooting, I know that shadowbox automatically searches for an adapter to use, so it must've switched to using the jquery adapter, which was failing for some reason.
so to fix it I set the adapter in the shadowbox init function to shadowbox's default, aka "base"
Shadowbox.init({
adapter: "base", <--- I added this line and it fixed itself
language: "en",
players: ["swf", "flv", "iframe"],
overlayOpacity: 0.85,
initialHeight: 400,
initialWidth: 640,
autoplayMovies: true
});