Ah! Finally found the culprit :-)

I run my fleet of runners in pre-emptible instances on GCP (they die after 24h).

I forgot to pin the version in my startup script, so all it did was a simple apt-get install gitlab-ci-multi-runner.

For some reason, version 9 of the gitlab runner cannot connect to our Gitlab (it fires a 404 on register).

Anyways, look like 9 is a cool release, but we won't be upgrading until the sidebar comes back (did a little poll in the team and it 100% was in favour of the sidebar)