I'll begin by apologizing if this topic has been addressed, but I've spent a good deal of time over several days trolling through Google hits and FAQ entries without success, so I turn here.
My environment is CentOS 5.2, Nagios V3.0.3. After the initial install, everything worked fine, including four network hosts in addition to my localhost entry (using NRPE). Then sometime in mid-December, I began getting the message in the subject line when I went to what was habitually my standard page, Unhandled Service Problems. The error message is displayed when I attempt to connect to any of the links under the 'Monitoring' header. Under the 'Reporting' header, Trends, Availability, and Alert Histogram also display the error; the other four links (Alert History, Alert Summary, Notifications, and Event Log) display data, but with a red textual warning in the upper left-hand shaded box that reads "Warning: could not read program status information". The 'View Config' link shows me all of my systems being monitored, but also with the red text warning just mentioned. Nagios entries in /var/log/messages are limited to localhost activity, but the events are being logged.
I've run nagios -v /etc/nagios/nagios.cfg and it returns with no errors. I've verified the selinux context settings on the directories as documented in the Fedora QuickStart installation guide (and even temporarily disabled selinux, with the same results). If this is an access or permissions problem, I (obviously) haven't found it, and I haven't been able to recall what changed between when the configuration worked and when it stopped working. (Unfortunately, I was in the middle of a customer crisis at the time and I didn't have (or take) the time to troubleshoot the problem logically when it first appeared.)
Thanks, and best regards,
/Harry Sutton, RHCA / RHCSS