Hello,
I'm afraid I don't read German, but this appears to be a problem with the MD5 checksum on the card you are using.
The webshop has been up and running and selling this week, and we have even sold some to Germany.
Which type of card are you using ?
The payment gateway is outside our control, the site takes you to the Worldpay gateway where the funds are taken.
I wonder if it is where you are not using one of the mainstream cards. The cards that are accepted are detailed on the payment gateway, are you using something else ?
kind regards
Dave.