Empty response of SOAP request

Empty response of SOAP request

I have two delphi clients of one ASMX service. One client is normal
desktop application and other is Outlook add-in. Everything works fine
(SOAP calls to ASMX service) on my PC. But on one of my costumers have
problems with SOAP calls within Outlook add-in, and at the same time
desktop application works as expected on the same machine.
The problem with SOAP call within Outlook add-in is error "XML document
must have a top level element". Reason of this error is empty response of
call. Take a look at logs:
8/7/2013-1:12:29 PM Response:
8/7/2013-1:12:29 PM XML document must have a top level element.
Line: 0
XMLDoc.TXMLDocument.LoadData + $2AA
XMLDoc.TXMLDocument.SetActive + $A8
XMLDoc.TXMLDocument.LoadFromStream + $29
Rio.TRIO.Generic + $70F
Response stream is retrieved in HttpRio AfterExecute method using call
fResponse.LoadFromStream(Response);
The question is: what is the reason of this error, how can one client on
the same machine work fine and other not? what can I do to reproduce,
diagnose this situation?
P.S. I know that it would be great to have sniffed http packets, but I
don't have access to the costumer's PC to run http sniffer.