I'm not using feodora,
but I think I can help.
First check what your admin account for nagios is.
Normally that is nagiosadmin (i think),
Then set the password for this accoutn using
htpasswd -c /usr/local/nagios/etc/htpasswd.users nagiosadmin
Then try logging in again,
If you're unsure what you admin account for nagios is you can check,
Ithink it's in template.cfg og hosts.cfg.