ASP.NET XML Parsing Error: no element found Line Number 1, Column 1 Error

That sounds like the Firefox error page that's returned when FF expects HTML and gets an empty document instead. Try looking at it with Firebug enabled and see what the Net tab says - perhaps you have a good header, but no html.

Usually that kind of thing is not due to an ASP.NET error (since with those you still have a document body for the Yellow Screen of Death page), but is more along the lines of a networking error...

Would it be possible to try looking at it from another computer to see if that's the problem?

To find the issue you are having with this problem.

In your global.asax file add:

void Application_Error(object sender, EventArgs e)
{
    Exception objErr = Server.GetLastError().GetBaseException();
    string err =    "Error caught in Application_Error event" +                
            "\n \nError Message: " + objErr.Message.ToString()+ 
            "\n \nStack Trace: " + objErr.StackTrace.ToString();

       System.Diagnostics.EventLog.WriteEntry("MYApplication", err, System.Diagnostics.EventLogEntryType.Error);
    Server.ClearError();     
} 

You can set a break point here or log this message into an EventLog.