What is the version and hotfix you are using? That page is virtual - The URL engine in Kentico routes it into a DLL file. However, the error you see usually means that the web server cannot access itself under the domain name/URL set for the admin app. E.g. if there is any proxy or load balance, firewall. You can try using web.config keys for scheduler URL (CMSSchedulerURL and/or CMSSchedulerAcceptAllCertificates) or, if you are running behind proxy, please see documentation on running behind proxy.