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 azarte.gitlab.io 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?
Thanks!