@laurent Just to be sure, did you check the same page again later? I’ve noticed the site going down for brief periods several times this year, so you may have been unlucky in timing.
Otherwise: I currently have no trouble accessing or displaying the page you linked. I have uBlock Origin, Firefox’s built-in protection, EFF’s Privacy Badger, and DuckDuckGo Privacy Essentials all enabled, with near-default settings. The only things blocked are cookies from YouTube and
stats.nextcloud.com, which appears to be a Piwik analytics server hosted on the same IP block as
www.nextcloud.com. Do you have any custom lists or settings enabled that might apply here?