I know with some update, i can’t tell which one it was, there was an update to the systemd-units that shipped with the agent and there the names were changed as well. At the least with the introduction of the cmk-agent-ctl this must have happened.
So it looks like with the update the check_mk.socket unit somehow still lingers around in your systemd-configuration, event though the socket-file does not exist anymore - as seen in the output Unit check_mk.socket not found
I would try the following: Check if there is a unit cmk-agent-ctl-daemon.service. The process started by this unit should be the one listening on port 6556 now instead of the “old” check_mk.socket. Then i would reload the systemd-configuration (systemctl daemon-reload) and if the only unit that is still marked as failed is the obsolete check_mk.socket i would just run systemctl reset-failed.
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