My group page is returning 503 error intead of 404

Hello everyone,
I am stuck with a minor problem that I can’t figure out. I am working in a new site for a GitLab group. The page is working correctly at but something strange happens when I ask for a page which doesn’t exist. Instead of loading custom 404.html file the browser shows GitLab sign in page.
Another group site is working as expected showing 404.html when I ask for a page which doesn’t exist. I check groups and projects configuration and I couldn’t see any differences.
Any clues?

This occurs because the group-level page domains use the path to point into their projects, and for security purposes any that does not automatically match a public project name will be subject to authentication before any response confirming existence/absence can be sent back.

This behavior likely doesn’t happen on actual project-level pages, which don’t have a nesting under them and will serve you actual 404s without needing to protect things under (when public).