Resolution:
Description
===========
This error is seen in the engine log file of TIBCO ActiveMatrix® Service Gateway(ASG), along with the following entries:
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2013 Oct 25 17:45:03:366 GMT -7 asg-caching-core Info [HttpWorker-3] - [user] [core-class] DEBUG: Received southbound response:
Error reading or parsing reply from server <host>:<port> from interface null Error=java.lang.NullPointerException
.......        
.......
2013 Oct 25 17:45:03:366 GMT -7 asg-caching-core Info [HttpWorker-3] - [user] [core-class] INFO: Response was error, code:7002
2013 Oct 25 17:45:03:398 GMT -7 asg-caching-core Error [HttpWorker-3] - [root] Error on line 1 column 1
  SXXP0003: Error reported by XML parser: Content is not allowed in prolog.
2013 Oct 25 17:45:03:398 GMT -7 asg-caching-core Error [HttpWorker-3] - [root] Recoverable error on line 10
  FODC0002: org.xml.sax.SAXParseException: Content is not allowed in prolog.
......
2013 Oct 25 17:45:03:398 GMT -7 asg-caching-core Error [HttpWorker-3] - [root] [Fatal Error] :1:39: Premature end of file.
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It is best to use tools such as Wireshark to capture the actual message being sent by the target service to determine the reason and take appropriate actions.

Resolution
===========
One possible reason is that the target service is sending an HTTP error in the response and there is no SOAP message. If the HTTP error is '404 Not Found', then the SOAPAction / URI configured in ASG is  wrong and needs to be corrected.

Resolving the error, "SXXP0003: Error reported by XML parser".