Problem with SAP NCO (Non-ABAP RFC client (partner type ) not supported)

Question

I have a problem with the SAP NCO 3.0. After the connection is established and the SAP System send an IDoc to my Server implementation, the NCO raised the following exception:

SAP.Middleware.Connector.RfcServerApplicationException: Uncaught exception SAP.Middleware.Connector.RfcUnsupportedPartnerException thrown in SAP.Middleware.Connector.RfcServer.ARfcDestShipImp when executing RFC server function ARFC_DEST_SHIP ---> SAP.Middleware.Connector.RfcUnsupportedPartnerException: Non-ABAP RFC client (partner type ) not supported at SAP.Middleware.Connector.RfcConnection.ReadUpTo(RFCGET readState, RfcFunction function, RFCID toRid) at SAP.Middleware.Connector.RfcConnection.Dispatch() at SAP.Middleware.Connector.RfcTransaction.Playback() at SAP.Middleware.Connector.RfcServer.ARfcDestShipImp(RfcServerContext ctx, IRfcFunction func) at SAP.Middleware.Connector.RfcServer.InvokeServerFunction(RfcConnection conn, ServerFunctionInfo serverFuncInfo, RfcServerFunction functionImp)

Function implementation:

class RfcFunctionImpl
{
    [RfcServerFunction(Name = "STFC_CONNECTION", Default = false)]
    public static void StfcConnection(RfcServerContext serverContex, IRfcFunction rfcFunction)
    {
        string dataFileName = string.Format("{0}.idoc", serverContex.TransactionID);
        FileStream dataFile = new FileStream(dataFileName, FileMode.OpenOrCreate, FileAccess.Write);
        StreamWriter writer = new StreamWriter(dataFile);
        IRfcTable iDoc = rfcFunction.GetTable("IDOC_DATA_REC_40");
        foreach (IRfcStructure record in iDoc)
        {
            for (int i = 0; i < record.Count; i++)
            {
                if (i == record.Count - 1)
                    writer.WriteLine(record.GetString(i));
                else
                    writer.Write("{0};", record.GetString(i));
            }
        }
        writer.Flush();
        dataFile.Close();
    }

    [RfcServerFunction(Default = true)]
    public static void GrDestAKLEllipta(RfcServerContext serverContex, IRfcFunction rfcFunction)
    {
        string dataFileName = string.Format("{0}.idoc", serverContex.TransactionID);
        FileStream dataFile = new FileStream(dataFileName, FileMode.OpenOrCreate, FileAccess.Write);
        StreamWriter writer = new StreamWriter(dataFile);
        IRfcTable iDoc = rfcFunction.GetTable("IDOC_DATA_REC_40");
        foreach (IRfcStructure record in iDoc)
        {
            for (int i = 0; i < record.Count; i++)
            {
                if (i == record.Count - 1)
                    writer.WriteLine(record.GetString(i));
                else
                    writer.Write("{0};", record.GetString(i));
            }
        }
        writer.Flush();
        dataFile.Close();
    }
}

I've tried different implementations, but nothing helps.

I hope someone can help me!

Regards Thorsten