UPDATE: Using the address given by SIXT in their reply below, I have managed to contact the customer service department and got my deposit back. But I think there is something seriously wrong about their procedures if I have to fight to get my deposit back - and about their customer care if there is no other way to reach them than a bad review on Google. :(
The rental went very well but I cannot have my deposit (500 EUR) back. I paid by a Vpay card - nobody told me I shouldn't, I would have had a Visa too. One day later I received a form to fill out with my bank details for the reimbursement, but it gave an error message and didn't work. I contacted Sixt several times on several channels to inquire how else I could give my account data for the reimbursement, but I only get automatic answers and nobody human actually begins to handle my case. So now, 11 days after the rental date, I still haven't been able to tell them how to pay my deposit back. This is unacceptable, Sixt is not allowed to keep my money indefinitely. They'd need to have (a) a process for reimbursements that actually works, and (b) a customer service that actually works. Failing that, I don't recommend them to anyone unless they want to part with 500 EUR.