I get this error when I try to call a webservice through a class generated from WSDL by WSDL2Apex. The strange thing is:

1. When I send the request to http://requestb.in, I can see that the request arrives properly. This is the message as seen in requestb.in:

<?xml version="1.0" encoding="UTF-8"?>
<env:Envelope xmlns:env="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
	<env:Header />
	<env:Body>
		<PingRequest xmlns="http://www.abacus.ch/abaconnect/2007.10/core/AbaConnectTypes">
			<Echo>Hello Abacus</Echo>
		</PingRequest>
	</env:Body>
</env:Envelope>

2. When I send the exact same message to the proper endpoint via SoapUI, I get a proper response:

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/">
   <soapenv:Body>
      <act:PingResponse xmlns:act="http://www.abacus.ch/abaconnect/2007.10/core/AbaConnectTypes">
         <act:Echo>hello this is the AbaConnect Webservice Server! Your message was: Hello Abacus( Ping-Nr: 1)</act:Echo>
      </act:PingResponse>
   </soapenv:Body>
</soapenv:Envelope>

3. But when I use Apex to send the request, I get the error message in the log:

19:08:10.249 (249851430)|SYSTEM_METHOD_ENTRY|[1846]|WebServiceCallout.invoke(APEX_OBJECT, APEX_OBJECT, MAP, LIST)
19:08:10.253 (253470025)|CALLOUT_REQUEST|[1846]|PingType:[Echo=Hello Abacus, Echo_type_info=(Echo, http://www.abacus.ch/abaconnect/2007.10/core/AbaConnectTypes, null, 1, 1, false), apex_schema_type_info=(http://www.abacus.ch/abaconnect/2007.10/core/AbaConnectTypes, true, false), field_order_type_info=(Echo)]::SFDC_STACK_DEPTH=1 SOAPAction="ping" User-Agent=SFDC-Callout/30.0 Accept=text/xml Content-Type=text/xml; charset=UTF-8 
19:08:10.344 (344790775)|EXCEPTION_THROWN|[1846]|System.CalloutException: Web service callout failed: Failed to get next element
19:08:10.344 (344950829)|HEAP_ALLOCATE|[1846]|Bytes:58
19:08:10.344 (344974228)|SYSTEM_METHOD_EXIT|[1846]|WebServiceCallout.invoke(APEX_OBJECT, APEX_OBJECT, MAP, LIST)

I searched and googled quite a lot, but couldn't find any solution so far. Some people have the same error, but no solution suggested that would help me.

Any help will be greatly appreciated.

Cheers

Mehmet