This error occurs if the application pool for the request does not exist, or if an ISAPI filter, ISAPI extension or HTTP ...

This error occurs if the application pool for the request does not exist, or if an ISAPI filter, ISAPI extension or HTTP module calls the ExecuteURL server support function (or ExecuteRequest) with a URL that is configured in a different application pool. Due to security reasons, a Web site in one application pool cannot make ExecuteURL requests against a URL in another application pool. If you have an application that is trying to process a URL in another application pool, ensure that they both run in the same application pool if appropriate.