There is a known error when your Nagios Monitoring Server is running Ubuntu 16.04 and you are trying to monitor a server with a newer version of Ubuntu like Ubuntu 18.04.
In Ubuntu 18.04, NRPE uses a 2048 bit DH key, but in earlier versions of NRPE use a 512 DH key is used.
This is a bit of a hack, but does work.
Firstly logon to an Ubuntu 18.04 server. If the nagios-nrpe-plugin package is not already installed, run the following command:
sudo apt-get install nagios-nrpe-plugin
Now copy the file /usr/lib/nagios/plugins/check_nrpe from your Ubuntu 18.04 server to your Ubuntu 16.04 NRPE server. For now copy it to your home folder.
Next logon to your Ubuntu 16.04 NRPE server. Now make a copy of the current check_nrpe file so you can restore it if you have any issues.
sudo cp /usr/lib/nagios/plugins/check_nrpe /usr/lib/nagios/plugins/check_nrpe.v2
Now you can copy the file you copied over into your home folder to replace the current check_nrpe file and restart the Nagios services.
sudo mv ~/check_nrpe /usr/lib/nagios/plugins/check_nrpe