Joerg,

I just went through your test order (17314981851355) and was able to update the address successfully for all other countries except Germany, so I went into the countries list and remove the > in front of the name and then Germany also began working when updating the address on the order. You can see the additional information I was able to add to that order's address.  Is there any particular reason for adding the > in front of Germany? 

Adam

Did the client want Germany to display as the default option in the list?

To set a default country or state/zone to display in the payment/shipping/billing fields, simply go to the Custom Fields sub menu under the Set Up menu and in your case you will want to click on the custom field for country_id. Then simply select Germany as the default country and that should take care of your client without causing any undo issues by adding characters in front of the actual country name. Let me know if this resolved that issue.