Nagios Support Forum

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5. Nagios installation

This step is most likely your problem. Did it actually complete properly? Is nagios running? Is Apache running?

It looks mostly like Apache doesn't know about the /nagios directory, which implies that Apache has not been restarted since the configuration changes done during Nagios install were made.

Could you also post/PM the last 20 lines or so of your http error log?
That "<" after the error message is a bit odd.