This stacktrace seems similar to what you are getting when the other application returns a SOAP Fault.
Can you check what the exact response is from the webservice you are calling? It might be that the other application is just returning a soap response of the type SOAP Fault, with the message: "Internal server error"
You can see the request/response messages if you enable the lognode: "Webservice", if you change that to 'debug' you can see all the messages in the log files.