Unable to authenticate using NAGIOS on redhat

Hi,

I have installed Nagios on Redhat 5 using rpms, it was successully installed and I have started the nagios and httpd service.

While login on browser, using nagios userid and password, I am unable to login

I have installed nagios and nagios-plugin rpms both.

I have used the command nagios -v /etc/nagios it is not giving any error or warning.

Please suggest