Seems to be an issue where your HTTP Provider is not able to process the PI Request and returns a Error Back.
- You can use a TCP Gateway to monitor the message that goes out of PI and the response that comes back and then check with the HTTP Provider team as to what is possibly wrong with the message. Steps to use TCP Gateway are as described here - Troubleshooting SOAP, HTTP and Mail Adapter Scenarios with TCPGateway
- Alternately you can use http://service.sap.com/sap/support/notes/2157425 to Enable Tracing on PI . Set Module Parameter TraceHTTP with value plain in your receiver HTTP_AAE adapter and you can see the logs in /server#/log/pi_http directory