What Is the Purpose of the Popup Permalink?
The "/popup/" links (permalinks) are not meant to be used for anything other than previewing the looks of your popups. They will not work for users that are not logged in with the ability to edit that popup. If you're not logged in, and don't have permission to edit the popup, then you will get a 404 error page when the link is clicked and/or accessed.
The reason we don't actually use the permalinks for anything other than previews is simply because popups are meant to be supporting content, not the content itself. This also means that popups cannot be tracked in analytics as a page view. If you are looking to track opens in Google Analytics we recommend our Popup Analytics Extension, which offers that ability, and much more.