This occurs if you have changed the DNS records so that the friendly URL that used to be assigned to that CMS site no longer point to Sphere.

To resolve this issue the old friendly URL must be unassigned in the site settings. Simply changing the DNS records will not remove the display URL from the site.

Because the display URL was not removed it will immediately redirect to the new site without executing the script and instead will try to load the page on the new site based on the page URL in the Sphere CMS resulting in the 404 error.

Chat with Support to request the removal of the custom domain from your CMS website. 

The analyst you work with will create a request to remove the custom domain from your site.  Once the request is processed, any secure page in your CMS website will display with the domain.