Sorry about that, link fixed, not sure what happened there...
Mattermost is designed to be upgraded one version at a time, so if you skipped versions using GitLab's automation, the Mattermost database schema won't matched the updated binary.
You can fix this by either re-running the GitLab automation to upgrade one version at a time, or you can manually upgrade Mattermost using the documented steps so the database schema matches the binary.
