If you still have some permission issue:
1. edit your cgi.cfg
and add the users to the lines where "nagiosadmin
" is, not all of them, it depend of the right you want to give to the user
2. Create the user and the password for the user you just add to your config file whithout the -c.
htpasswd2 /usr/local/nagios/etc/htpasswd.users user2
3. Restart apache.
I've used nagios for years now, and I always did like this.
PS: For security reason I would recommend to remove the nagios admin user from your config file