jQuery Grid Plugin - jqGrid

We are using jqGrid for a couple of screens in our asp.net application and it works well until we try using the method ShowEditValidationMessage("Error Message here").  This should show the message at the head of the dialog. It works fine on a development PC or a simple and relatively unsophisticated web server with no security, but when it's deployed in a production environment with standard security set up, when this method is called, it causes 500 server errors.  We cloak errors here - substituting all kinds of errors (404,500 etc)  with custom errors so that we don't potentially display internal error information or stack dump information - the kind of information valuable to hackers etc.  But this means we don't get the contextual error message in the dialog.  It now just says 'error'. This is no use to the person who has a dialog box they are trying to fill in with no clue as to what has happened. 

There are two or three ways forward.  We have tried storing the error that should appearion in the session object then reading this object again in the custom page for the 500 server error but the session data is not present as it's on a different thread.  Secondly we have tried looking into changing the word 'error' with something else but this word error cannot be changed (I've tried substituting something else in the jqGrid resource file).  We are next embarking on building some custom validation to run before 'submit' is clicked thereby anticipating the error and getting the user to change it before submission occurs (which would then show cause the problem), whilst submit is rendered disabled until all error conditions are fixed.  However the latter course is taking a lot of time and energy that is effectively a workaround for a fundamental error in jqGrid.  Can we just have the functionality that ShowEditValidationMessage() should provide without the 500 server error problems?