Having the same issue with a SOAP channel on 7.5
The messages work fine in 7.1 but the Java SOAP adapter is getting
java.io.IOException: Parsing Error: org.xml.sax.SAXParseException; lineNumber: 1; columnNumber: 1
And when looking in Configuration and Monitoring, the payload is empty - all the content has been lost.
I changed the same SOAP adapter to check the Do Not Use SOAP Envelope parameter, and add nosoap=true to the URL and the channel works if you send the same request via SOAP UI without the SOAP envelope. Don't forget to move any required namespaces from the Envelope to your XML however. If I find how to get this working with a SOAP envelope I will update this.