On the server you can remove xinetd (with check-mk) and immidate reinstall check-mk after that.
Be sure, that you have a good backup, first 
.
On linux hosts the agent deactivates xinetd configuration automatically, but we saw still xinetd services running which prevent the agent-service from starting.
systemctl
should list the services in “running”
cmk-agent-ctl-daemon.service loaded active running Checkmk agent controller daemon
check-mk-agent-async.service loaded active running Checkmk agent - Asynchronous background tasks
Did you tried the steps descriped here https://docs.checkmk.com/latest/en/agent_linux.html#test ?