CRIT - no unmonitored services found, no vanished services found, no new host labels, [agent] Communication failed: [Errno 111] Connection refusedCRIT

Seems like it’s a problem persisting between all versions and no final fix available. You can work around by running systemctl reset-failed check-mk-agent@*. But’s only a temporary fix if one of the agents fails.
Reason behind this issue is a not properly ended agent process and systemd keeps the unit active but failed.
Related posts i found (partially in german):