Jens🇩🇪 I already contacted Mastercard a few times. Unfortunately, they point to Sixt or the company that handles their payments.
I also tried to get an answer from Sixt... but they refuse to answer and they don’t have a complaint procedure apparently. But I found a clue in their terms & conditions. They state that the down accept prepaid cards and mention bunqs Travel Card explicitly. But, officially the Travel Card is an normal credit card. I have pointed that out to Sixt, but they are just love to ignore people. I know bunq also is trying to get in contact with them.