Can't register a ci-runner

I imported a backup from a hand-installation of GitLab (it had been set up in 6.x as far as I remember, and upgraded since then) in GitLab Omnibus via the Docker image. Everything's working fine, except one thing: I can't register a GitLab-CI runner. I point them to http://mygitlab.com/ci, with the token I can find on the admin/runners section. I end up with a 404. I can see the request made in GitLab's log, without any details, but ending here in a 404. If I omit the /ci part like I did at first, I end up with a 405.

I tried to set-up a new GitLab instance with the same Docker image and settings, it works fine. So I guess there's some setting which imported incorrectly from the backup, but I can't find anything relevant.