Error in product registration process, what can I do?

Hi,

after purchasing a Mac pro in a third party reatil store (Germany), I tried to register the product on my support page. But after entering the serial number, all i got was the message "Leider scheint das Kaufdatum für die eingegebene Seriennummer nicht korrekt zu sein" (transl: "Unfortunatley the purchasing date seems not to be correct in relation to the serial number"). And so I got stuck there ...

Why will this message occurr?

What may I do to register my Mac pro?

Thx!