Unauthorized access at ASP.global.asax file in webforms application

the application throw 500 runtime error. And the after some time the application starts working fine. After fetching the event logs we found the below mentioned unhandled exception.

Process information:    Process ID: 75788     Process name: w3wp.exe     Account name: IIS APPPOOL\XX Exception information:    Exception type: HttpException     Exception message: Unauthorized access   at ASP.global_asax.Application_PreRequestHandlerExecute(Object sender, EventArgs e)   at System.Web.HttpApplication.SyncEventExecutionStep.System.W开发者_JS百科eb.HttpApplication.IExecutionStep.Execute()   at System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step)   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Please Note without any changes in the application or at server level it starts working again after some time.

We have checked, proper access rights have been provided to the Application folder mapped on the IIS.