Hi Steven,
You can restart the service by running:
systemctl restart check_mk.socket
Also, you may want to check if there are hanging agent processes:
ps auxwww | grep check_mk
If the last command returns several check_mk_agent processes you may want to check if perhaps some took a very long time to complete and are using up the maximum number of available sockets for the check_mk agent (I believe the default is 3).
systemctl --failed
should have no output.
Kind regards,
Louis.