In the /etc/nagios3/nagios.cfg file, make sure you have similar values:

<code>
check_external_commands=1
command_check_interval=15s
command_file=/var/lib/nagios3/rw/nagios.cmd
</code>

Add www-data user to nagios group

<code>
usermod www-data -G nagios
</code>

<code>
dpkg-statoverride --update --add nagios www-data 2710 /var/lib/nagios3/rw
dpkg-statoverride --update --add nagios nagios 751 /var/lib/nagios
</code>

Restart Nagios