Correct, upgrades should go ESR → ESR
I have a feeling that making a big jump that skipped ESR may have wound up corrupting or copying an incompatible plugin.
I think you can look in /opt/mattermost/prepackaged_plugins and see if there are old ones, permissions issues that prevented an upgrade, etc. They should be deleted and replaced during an upgrade.
Did you start with the Teams (free) version? You may have oerwritten that with the licensable version, which would also cause issues. If you start with Teams, you need to make sure you’re following the upgrade with newer Teams versions.