I recently had to upgrade older Ubuntu distributions from 18.04->20.04->22.04->24.04 which had Nagios running on them.

Between 20.04 and 22.04 libssl has had a version change which breaks Nagios from starting once the OS has been upgraded. This is a pretty common problem with lots of different software breaking because of libssl after upgrades.

Nagios specifically breaks with:

/usr/local/nagios/bin/nagios: error while loading shared libraries: libssl.so.1.1: cannot open shared object file: No such file or directory.

The easiest way to recover from this is to just rebuild from source:

./configure --with-nagios-user=nagios --with-nagios-group=nagcmd --with-openssl
make all
sudo make install
sudo make install-init
sudo make install-commandmode
sudo systemctl restart nagios
sudo systemctl status nagios

Apache also requires new modules for PHP as 8.1 with 22.04 and 8.3 with 24.04 was introduced during the upgrade. That change is pretty straight forward to enable support for it in Apache:

sudo a2enmod php8.1

or

sudo a2enmod php8.3