Thanks for the reply.
It's on Debian, but I finally found the problem: there was a `valid users` directive in the smb.conf file.

Hello,

I followed the tutorials (very good, by the way) to install Samba4 as an Active Directory server:
https://dev.tranquil.it/wiki/SAMBA_-_Installation_d'un_AD_Samba4_pour_un_nouveau_domaine
and
https://dev.tranquil.it/wiki/SAMBA_-_Installation_d'un_nouveau_serveur_de_fichiers_Samba4
for the server of...