Some components, like pipelines (CI/CD), do not work after the upgrade. We're looking into it.
Update 2019-01-11 11:32: The outage was caused by a failed database migration. This, in turn, was caused by the upgraded Redis daemon listening on a different UNIX socket. The latter was resolved quickly after the GitLab update to 11.5, but the former was not spotted due to the large amount of output produced by the GitLab upgrade.