If both supplier and customer are located in Germany, BT-124 "External document location" must contain an absolute URL with valid scheme.
(/ubl-invoice:Invoice/cac:AdditionalDocumentReference/cac:Attachment/cac:ExternalReference | /ubl-creditnote:CreditNote/cac:AdditionalDocumentReference/cac:Attachment/cac:ExternalReference)[$supplierCountryIsDE and $customerCountryIsDE]matches(cbc:URI, $XR-URL-REGEX)