UPDATE: The following has been confirmed by prelado.de
I send them a query by mail. Here is their reply:
Leider können wir aktuell aus Sicherheitsgründen nur Kreditkarten deutscher Banken für das Aufladen des Guthabens akzeptieren. PayPal konnte bisher auch von ausländische Kunden erfolgreich genutzt werden.
Translation: Unfortunately, we can only accept credit cards of German banks for top-ups right now for security reasons. PayPal can be used sucessfully by foreign customers for topping up.
So summing up: German SIM cards are still hard to reload by non-German credit cards. Only Ortel and Lebara offer this option on their website (but may restrict some cards too). 3rd-party reload sites like prelado.de only accept German credit cards.
But there is a loophole: If you want to keep a German SIM card alive and are not in the country (where you can buy vouchers), use PayPal e.g. on
www.prelado.de without surcharge. Only for transactions of NON-€ currencies PayPal charges high conversion fees.