Are you calling a Mendix webservice? There is a bug in Mendix that when you have a published webservice that has validate against wsdl turned on it can crash. This crash will then cut off the response stream and result in an invalid response. To fix this you have to be sure that the response that is generated on the publishing side matches the webservice definition e.g. the following example will result in this situation.
<Response> <Code>1</Code> <!-- No description provided/target mapping is empty --> <Response> <element name="Response"> <complexType> <sequence> <element name="Code" type="String"/> <element name="Description" type="String"/> </sequence> </complexType> </element>