mapping error in SOAP --> SFTP integration flow

We set up the following integration flow in SAP PI:

SOAP Adapter as synchronous interface should receive message which should be transfer to a SFTP server. The message should be mapped by an SAP PI mapping to the defined output message format. Attached you can see a screenshot of this integration flow..

Sending a message to the webservice interface of this PI scenario we receive the following error message while the message has been delivered to the SFTP server !!!

com.sap.engine.interfaces.messaging.api.exception.MessagingException: com.sap.engine.interfaces.messaging.api.exception.MessagingException: Error encountered while executing mapping: com.sap.aii.af.service.mapping.MappingException: com.sap.aii.utilxi.misc.api.ResourceException: Could not determine mapping steps for message cf4cba8b-8919-11e5-8ac2-000006276b3a

at com.sap.aii.adapter.soap.web.SOAPHandler.processSOAPtoXMB(SOAPHandler.java:772)

at com.sap.aii.adapter.soap.web.MessageServlet.doPost(MessageServlet.java:530)

For this test we are using the webservice client in the SAP Web Services Navigator.

If we remove the message mapping from the iFlow the webservice call will processed without any error and we are receiving as answer from the webservice the directory and the filename of the delivered message. For sure the message is delivered to SFTP in the original message format.

So it seems that the receiver adapter sends a message back which can not be processed in the message mapping.

Can somebody help me ?