In IIS10,

We have a custom error application (CustomError) that is running under an application pool (CustomErrorAppPool). 

We have few more web application, For example RootSite running under seperate application pool (RootSiteAppPool). 

I have configured Error pages for 404 error code in RootSite Application, pointing to /customError/ErrorPage.asp using ExecuteURL option. 

when I browse http://www.RootSite.com/invalidpage. It shows 403.18 as below.

I have checked the FailedRequestLogs for  RootSite Appication, It successfully route the error code to CustomError Applicaiton, but the CustomError Application failed to execute the error page and throws the below error. 

When the RootSite and CustomError sites runs in single/same AppPool, the error does not occurs, and it display the ErrorPage.asp page content. 

The error occur only when the sites run in different Application pool. I have 100+ sites in my IIS server, I cannot configure all of them to run in single/same AppPool, which is not recommended. 

Please help me to fix the error. 

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<div id="error-information-popup-container" jstcache="0"> <div id="error-information-popup" jstcache="0"> <div id="error-information-popup-box" jstcache="0"> <div id="error-information-popup-content" jstcache="0"> <div class="error-code" jscontent="errorCode" jstcache="18">HTTP ERROR 403</div> </div> </div> </div> </div>