Hi @lclaude,
I tried it again and it seems to be a problem with joomla and virtualhosts. If you put your joomla site to /var/www/html, which is the NethServer default web site location, it works with HTTPS. This could be a solution for you as your other sites work as vhosts.
But I am still interested, why joomla is not working with virtualhosts so I uncommented the entries in /etc/httpd/conf.d/virtualhosts.conf and Joomla .htaccess but no success.