Sometimes you run into this error when trying to access the web interface. Usually the ToC on the side will load, but any link gives a 500 error page. In my case, this is what did the trick.

Example of errors in the http error_log:

[Fri Feb 17 02:41:37 2006] [error] [client 192.168.1.50] (13)Permission denied: exec of '/usr/lib/nagios/cgi/statusmap.cgi' failed, referer: http://192.168.1.21/nagios/side.html
[Fri Feb 17 02:41:37 2006] [error] [client 192.168.1.50] Premature end of script headers: statusmap.cgi, referer: http://192.168.1.21/nagios/side.html
[Fri Feb 17 02:41:38 2006] [error] [client 192.168.1.50] (13)Permission denied: exec of '/usr/lib/nagios/cgi/status.cgi' failed, referer: http://192.168.1.21/nagios/side.html

Use the following to fix:

chcon -R system_u:object_r:httpd_sys_script_exec_t /usr/local/nagios/sbin