Comment 4 for bug 441444

I can confirm this, as it's happened to me also.

It seems to be when you've timed out an authorisation to last.fm, and therefore the URLs that it's trying to download are no longer valid, and therefore you get redirected (or shown) a 404 page. Rythmbox then tries to download this as a media file.

If last.fm is returning 404's, then rhythmbox should realise this. If it's not, then I can probably poke the devs as I know a couple of them and get them to potentially fix it their end