[Axis2]: org.apache.axiom.om.OMException: Unable to locate the SOAP Part

We use an Axis2 (1.6.1) webservice and client (top-down design using ADB) to collect medical images (using MTOM).

While this works smoothly all over the world, a customer in Turkey can not get rid of the following exception:
(...)

Caused by: org.apache.axis2.AxisFault: Unable to locate the SOAP part; content ID was [email protected] <mailto:[email protected]> 

 at org.apache.axis2.AxisFault.makeFault(AxisFault.java:430)
 at org.apache.axis2.transport.TransportUtils.createSOAPMessage(TransportUtils.java:123)
 at org.apache.axis2.transport.TransportUtils.createSOAPMessage(TransportUtils.java:67)
 at org.apache.axis2.description.OutInAxisOperationClient.handleResponse(OutInAxisOperation.java:354)
 at org.apache.axis2.description.OutInAxisOperationClient.send(OutInAxisOperation.java:421)
 at org.apache.axis2.description.OutInAxisOperationClient.executeImpl(OutInAxisOperation.java:229)
 at org.apache.axis2.client.OperationClient.execute(OperationClient.java:165)
 at ch.msmri.transfer.client.DicomTransferServiceStub.datasetUpload(Unknown Source)
 ... 11 more
Caused by: org.apache.axiom.om.OMException: Unable to locate the SOAP part; content ID was [email protected] <mailto:[email protected]> 
 at org.apache.axiom.attachments.Attachments.getSOAPPartContentType(Unknown Source)
 at org.apache.axis2.builder.MIMEBuilder.processDocument(MIMEBuilder.java:41)
 at org.apache.axis2.transport.TransportUtils.createDocumentElement(TransportUtils.java:179)
 at org.apache.axis2.transport.TransportUtils.createSOAPMessage(TransportUtils.java:145)
 at org.apache.axis2.transport.TransportUtils.createSOAPMessage(TransportUtils.java:108)
 ... 17 more

Not sure if the following info is of any help:
- all communications run via https
- Client runs on Windows XP/JRE 1.6.0_29-b11
- The customer's institute uses no proxy server
- The customer's institute uses a "Trend micro security agent", however added an exception for connections to our webserver (to exclude as possible source)

Does anyone have an idea what could cause this exception?
Any help is highly appreciated
Thanks in advance
--stefan