If you may receiving the following errors from the Nagios alert, 

***** Nagios *****
Notification Type: PROBLEM
Service: Current Users
Host: hostname.com
Date/Time: Thu Jun 23 06:29:05 EDT 2016
Additional Info:
(Return code of 255 is out of bounds)

You need to check in two ways, start the xinetd service and need to add the server ip address in remote host nrpe.cfg file.

# service xinetd restart

xinetd stop/waiting
xinetd start/running, process 2377

Also, you need to check with nrpe file,

# vim /etc/xinetd.d/nrpe

        only_from       =