UCS 4.3.3 errdata390 + Nagios 4.3:
Nagios was never installed before.
After 1st installation of the Nagios server on an UCS Member server (!) and the Nagios clients on the other server, the config directive “allowed_hosts” contain always (on all servers) and only the FQDN from the UCS AD Master.
Therefore, the NRPE connection errors are initially displayed.