I see that the post is really old but still wanted to post a solution which is not perfect but still a workaround
I was also getting the same error and the fix which worked for me was
Code:
# chmod 777 /usr/local/nagios/var/rw/nagios.cmd
NOTE: Do not Restart nagios as after restart it changes its permission to default one and I am still looking for a permanent solution for this.
