They didn't really say anything. They just said that my invoice was processed with errors. When I checked my billing account the invouice was still open and it said "action blocked".
My card is still valid, since I just got my credit card bill and they booked it from my bank account just three days ago.

I wrote a mail and asked if I should pay again or if they will try and solve it. It might be a connection falure to the bank computer. I have no idea. They didn't say I should pay again, just that I shoiuld check and probably change my credit card number (which I can't since I only have this one).

No answer yet, so I don't know.