In addition to what
@tgriep said, did you see the suggestion
@ssax made? Here it is again:
Please validate you have performed the 3 steps listed on page 3 here:
https://assets.nagios.com/downloads/nag ... s%20XI.pdf
You will at least need to change the
$cfg['use_https'] setting in your /usr/local/nagiosxi/html/config.inc.php but also validate the CCM settings as well under Configure > Core Config Manager > CCM Admin > Settings > Paths tab.
Also, what is it you are installing on Windows? If it is NSClient, please let us know the version.