The stock advice from the company has been this: "Why am I having problems ordering with a credit card?
Please be aware that our online Webstore is based in Germany and requires an international payment. If there are problems with ordering, then we recommend to check with your card provider and make sure that they allow international payments. If that does not help, then we recommend to try a different card or payment method, like Paypal."
You might email infoservice@magix.net or ask at Online Live Chat (advice in section 6 here).