Of course i will tell you
My problem was for munin. Without virtual host, I had to find where it was configured.
I tried this directive in the /var/www/munin/.htaccess file :
Deny from all
But it wasn't working.
I found that the configuration file for this was /etc/apache2/sites-enabled/000-ispconfig.conf, so in this one, I had :
Allow from all
then restart apache : invoke-rc.d apache2 restart
I had this file : /var/www/munin/.htaccess :
AuthName "Members Only"
<limit GET PUT POST>
Creation of the password :
htpasswd -c /var/www/munin/.htpasswd admin
And everything was working