This error is caused by the server not being able to request a copy of the page being cached. This error often occurs in a production environment where the server is locked down.
Resolution
Option 1. Open a browser and check that you can navigate to the page using the external url. If the external url is not available, modify the server configuration so that it can call the page from the external URL.
Option2. Modify the code so that the URL property requests and internal version of the page eg http://localhost/page.asp