Error: Could not stat() command file '/var/lib/nagios3/rw/nagios.cmd'!
I ran into this error while messing around with nagios. Manually playing around with on /var/lib/nagios3/rw/nagios.cmd and its containing directories didn't seem to help much, so I went a-googlin'.
Here is the solution.
In short: (in case that blog goes down and I need it again ;-) ) :
sudo /etc/init.d/nagios3 stop
sudo dpkg-statoverride --update --add nagios www-data 2710 /var/lib/nagios3/rw
sudo dpkg-statoverride --update --add nagios nagios 751 /var/lib/nagios3
sudo /etc/init.d/nagios3 start