Hello Dirk Goeritz, I understand how frustrating it must be to encounter this issue despite taking all the right troubleshooting steps, especially when you're trying to get your device repaired promptly.
One thing you can try is going to your Microsoft Account Profile and making sure your billing address, language, and region are all correctly set to Germany. Also, double-check that the repair portal itself is set to Germany this can usually be confirmed by looking at the URL or the region setting in the footer of the page.
If everything looks correct, try removing and re-adding your payment method to refresh the connection. This has helped some users resolve similar issues.
Kind regards,
John Oli