I run into this from time to time.
The easiest solution is to ensure that your /etc/xinetd.d/nrpe ‘only_from’ and your nrpe.cfg ‘allow_from’ are configured properly. This doesn’t always cut it.
If you are attempting to add a new server to an existing Nagios architecture, have verified everything is solid, and still get this error:
Check your NRPE version
Assuming you have other servers where NRPE is working fine, try using the same version they have. Ran into this today. It should NOT have taken 2 hours to figure out. Just another day in the life.