When developing Web services applications in 4D it is important to remember that any possible errors that occur in 4D can be returned as a SOAP Fault if you do not have your own error handling in place.
For example a Web service call might return "SOAP Fault #53". The "53" in this case is literally the 4D error code 53, "Indice out of range.":
When a Web service is called 4D executes the On Web Authentication (OWA) database method and then the Project method that has been published as a Web service. If an error is generated during the execution of either of these methods, 4D ends the SOAP connection and returns the error code as a SOAP Fault. Using the example mentioned above, this would mean that either OWA or the Web service method (or any code called by those methods) encountered an un-handled 53 error.
If you do not want to let 4D return this error as a SOAP Fault, you must define an error handler using the ON ERR CALL command, so that you can return your own SOAP Fault or ignore the error.