While debugging my check disk problem after the 15.10 upgrade, I saw that I forgot to enable external commands. That is handy, when you want to re-schedule a check to see, if your changes took effect. Again, something that is easily activated. So if you see something like this, then you might want to make some changes:
Error: Could not stat() command file ‘/var/lib/nagios3/rw/nagios.cmd’!
The external command file may be missing, Nagios may not be running, and/or Nagios may not be checking external commands. An error occurred while attempting to commit your command for processing.
First stop the Nagios service with systemctl, service or with the init script. Whatever your distribution prefers. Then edit as root the configuration file /etc/nagios3/nagios.cfg and check if the variable check_external_commands is set to 1:
Afterwards update the rights to the external command with the following:
dpkg-statoverride --update --add nagios www-data 2710 /var/lib/nagios3/rw
dpkg-statoverride --update --add nagios nagios 751 /var/lib/nagios3
And then start Nagios again. Et voila, you can call external commands.