The client was unable to process the callback request due to an internal error. For more information about the error, either ...

The client was unable to process the callback request due to an internal error.  For more information about the error, either turn on IncludeExceptionDetailInFaults (either from CallbackBehaviorAttribute or from the  configuration behavior) on the client in order to send the exception information back to the server, or turn on tracing as per the Microsoft .NET Framework SDK documentation and inspect the client trace logs.