When Return Form (Beilegeretoure) is enabled in Magento, the following error occurs when trying to create labels:
"hard validation error occured. | der angegebene service ist nicht bekannt."
If Return Form is disabled, the label can be created normally.
Most likely, this is a misconfiguration in the DHL account, i.e. Return Shipments are not activated at DHL.
Contact the DHL team and ask them to check and correct the settings in your DHL account.