RFC Receiver channel: failed to read funtionname from XML

Dear experts,

I have a scenario:

IDoc --> RFC --> Flat file

I get an IDoc (PPCC2RECORDER01) from the ERP system via IDoc adapter into SAP PI 7.3.

I map this to an XML structure and hand it to an RFC receiver adapter that uses "MessageTransformBean" to convert the

XML to a flat file structure.

Now in Runtime Workbench I get the following error:

"MP: exception caught with cause com.sap.engine.interfaces.messaging.api.exception.MessagingException: com.sap.aii.adapter.rfc.afcommunication.RfcAFWException: error while processing message to remote system:com.sap.aii.adapter.rfc.core.client.RfcClientException: could not get functionname from XML requst: com.sap.aii.adapter.rfc.RfcAdapterException: failed to read funtionname from XML document: missing namespace declaration(2)"

I added the RFC namespace in my mapping and the target XML before flat file conversion looks like this:

<?xml version="1.0" encoding="UTF-8"?>

<ns0:resultset xmlns:ns0="urn:sap-com:document:sap:rfc:functions">

    <row>

        <EDI_DC40>

    ....

    </EDI_DC40>

    </row>

    <row>

        <E1BP_PP_PDC_OPERA2>

    ...

        </E1BP_PP_PDC_OPERA2>

    </row>

</ns0:resultset

It looks good to me! Why do I get an error in the Runtime Workbench saying "missing namespace declaration(2)" ??

Thank you for your help with this,

Peter