@Tgr: Ah, thanks for the info. I affected all three of my uploads that I tried to use on de.wikipedia today, hence I thought it was really very urgent.

I think there should be at least a Commons site notice to warn of this crucial bug ("Priority: normal" must be a joke, right?). It's a really bad user experience to upload stuff and then not be able to use it on the home wikipedia then, without knowing why at all. It took me half an hour to find this bug report...