Hi,
How are you connecting to the server? Is that via xinetd, the systemd socket or ssh? If it’s one of the first two you may want to try and restart that service. A reinstall of the agent should be a bit drastic solution.
Also, see if you can get a connection from the Check_MK server to the client on port 6556:
nc -v <your server IP> 6556
If that fails, try it from the client, using localhost for the server IP.
Louis.