The GitLab version I was running was not the latest one: it was the 8.15.1-ce.0 while the 9.2.1-ce.0 has been available for a few days now. So I decided to upgrade. But I faced a problem: a 500 error page was displayed after the upgrade.
Looking at the logs, it appeared that error was related to Postgresql.
So, I was able to correct the problem with following procedure:
gitlab-ctl revert-pg-upgrade
gitlab-ctl pg-upgrade
gitlab-ctl status # shows that postgresql is down
gitlab-ctl start postgresql
And the error page vanished.