Support

Check your configuration.php file. You should have

public $live_site = '';

If the $live_site is not empty, make it empty. I believe that it reads something like

public $live_site = 'http://www.example.com';

which tells Joomla! to always redirect from HTTPS back to HTTP. The HSTS setting tells the browser to always redirect from HTTP to HTTPS. You see the problem now? The browser goes from HTTP to HTTPS. The server tells it to go back to HTTP. The HSTS setting tells it to go back to HTTPS. The server says no, dammit, go back to HTTP. And so on and so forth until it happens for 20 times and the browser goes "You know what, guys? Sod that! I can't display this page because there are too many redirects."