Fixing Nagios Error: It appears as though you do not have permission to view information for any of the services you requested

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I recently set up a new Nagios installation and was dumbfounded by the following error:
It appears as though you do not have permission to view information for any of the services you requested...

If you believe this is an error, check the HTTP server authentication requirements for accessing this CGI
and check the authorization options in your CGI configuration file.

This was happening only occasionally, so I was confused about why this would be...
I was using Debian 5.0.4 ("lenny") with Nagios 3.2.1 and Nagios Plugins version 1.4.14. The installation directory is /usr/local/nagios.

What I found after much troubleshooting is that Nagios was already running after I had set it up as a fresh install. I had upgraded to Nagios 3.2.1 from 3.2.0 and the /etc/init.d/nagios restart command had not been ending the old Nagios process. This meant I ended up with strange entries in my logs like:

[1268348749] Warning: The check of service 'PING' on host 'switch1' looks like it was orphaned (results never came back).  I'm scheduling an immediate check of the service...

After running a ps aux |grep nagios and seeing the multiple processes it was obvious that the old process was interfering with the new ones. I hope this article saves you some time!