Problem
When Return Form (Beilegeretoure) is enabled in Magento, the following error occurs when trying to create labels:
If Return Form is disabled, the label can be created normally.
Cause
Most likely, this is a misconfiguration in the DHL account, i.e. Return Shipments are not activated at DHL.
Solution
Contact the DHL team and ask them to check and correct the settings in your DHL account.