ASP.NET 2010 path

Question

I am looking at a C# asp.net web forms 2010 application to see how to add some new webpages to the main selection page uers see. (I have C#.net desktop experience, but I do not have C#. asp.net expereince).
 
Right now I am looking at code that works when a user selects a web page that is working versus a web page that is not working.
 
The application is looking for a file in the http://localhost/path/app.aspx. However I do know know where this logic is that looks for the file listed above. is there another web.config and/or machine/config file i need to look for that. What file would i be looking for?
 
the code in Application_AuthenticateRequest seems to work ok. Then the next thing that happens is
 Application_Error is called from ASP.global_asx as object sender and the eventsArgs e is empty.
 
I do not know how the Application_Error is even called.
 
 
 
When the error occurs, the following scenarios happens:
 
The global.asax.cs has the following code:
  protected void Application_Error(object sender, EventArgs e)
  {
  Exception objErr = Server.GetLastError().GetBaseException();
  string err = "Error Caught in Application_Error event\n" +
  "Error in: " + Request.Url.ToString() +
  "\nError Message:" + objErr.Message.ToString() +
  "\nStack Trace:" + objErr.StackTrace.ToString();
 
  try
  {
  EventLog.WriteEntry ("Sup", err, EventLogEntryType.Error);
  Server.ClearError();
  Response.Redirect("~/ErrorPage.aspx");
  }
  catch (Exception ex)
  {
  }
  }
  void Application_AuthenticateRequest(object sender, EventArgs e)
  {
  // Extract the forms authentication cookie
  string cookieName = FormsAuthentication.FormsCookieName;
  HttpCookie authCookie = Context.Request.Cookies[cookieName];
  if (null == authCookie)
  {
  // There is no authentication cookie.
  return;
  }
 
  FormsAuthenticationTicket authTicket = null;
  try
  {
  authTicket = FormsAuthentication.Decrypt(authCookie.Value);
  }
  catch (Exception ex)
  {
  // Log exception details (omitted for simplicity)
  return;
  }
  if (null == authTicket)
  {
  // Cookie failed to decrypt.
  return;
  }
  System.Security.Principal.GenericIdentity id = new System.Security.Principal.GenericIdentity(authTicket.Name, "LdapAuthentication");
  // This principal will flow throughout the request.
  System.Security.Principal.GenericPrincipal principal = new System.Security.Principal.GenericPrincipal(id, null);
  // Attach the new principal object to the current HttpContext object
  Context.User = principal