@tosch TCP 6556, not UDP.
@Alexander do a netstat -tnlp (TCP, Numbers, Listening, Program/PID) to see what’s listening at 6556 already. Then kill it 
I suspect it will be the systemd listener, since I see check-mk-agent.socket enabled in you output there.
FWIW the only time I had this sort problem was also that the systemd listener was also installed (and for whatever reason wasn’t working - I’ve always had bad luck with it…).
reset-failed won’t fix anything, it just clears the failed flag on dead services so that systemctl status says the system state is clean. You’ll wanna systemctl stop check-mk-agent.socket; systemctl disable check-mk-agent.socket (and possibly that check-mk-agent@.service also, I don’t think I’ve seen that before, doubt you’ll need it using xinetd). You should then be able to start xinetd just fine.
Let me know how that goes.