It seems that deleting et creating again the user is enough to fix the FTP connexion problem.
I've modified my global apache configuration with the following :
DirectoryIndex index.html index.htm index.xhtml index.php index.txt
ServerName debian.domain.tld
#ServerName localhost
HostnameLookups Off
ServerAdmin myadressemail
UserDir www
UserDir disable root
<Directory />
Options -Indexes FollowSymLinks
AllowOverride All
</Directory>
ServerSignature Off
An now my .htaccess is :
AuthUserFile /home/file1/myfile/.htpasswd
AuthGroupFile /dev/null
AuthName "My authentification"
AuthType Basic
Require user user1
But I still have got no authentication asked, what did I do wrong ?