Not sure if I should just reply to an old post, but I have the exact same issue and it does work fine if I use IP instead of hostname as suggested - problem is there is no stated reason/solution for this. So how do I get it to work with hostname again?
Note this started for me when I added "Redirect permanent / https://<Your-XI-Server-Address-or-Hostname>" to /etc/httpd/conf.d/nagiosxi.conf as per page 3 of "How to Configure SSL for Nagios XI" document, but removing this (and restarting httpd) does not help.
Any assistance appreciated as search is not finding anything for me.
I would make sure the host name that you are using is setup in your DNS server and that both the server and client can do a lookup of the hostname correctly.
Also, try clearing out the cache and cookies for your browser to see if that helps.
Additional information.
I had issues with Internet Explorer not redirecting, I changed the following and it seemed to resolve it.
Put a trailing forward slash on the end of the redirect like below and post back if it works for you.