So the web service can be called and returns the correct data - you just can't access the top-level nodes attributes?
If that is the case you could get them via the PAYLOAD protocol (IF_WS_PAYLOAD).
i.e.
payload_handler = cl_proxy_access=>get_payload_handler(
proxy_name = 'CO_...'
proxy_method = 'EXECUTE_SYNCHRONOUS' ).
Then use "GET_PAYLOAD_FROM_RESPONSE_DATA()".
* Show payload for test purposes
lo_pointer = lo_payload->get_xml_pointer( ).
cl_proxy_service=>show_xml_document( lo_pointer-> ).