OK, isso foi um bug, o problema foi resolvido quando instalei o samba 4.3.5 (eu o reconstruí a partir do código fonte). E agora tudo funciona como um encanto!
Muito obrigado ao Grawity por me guiar para esta solução.
Upgrade to Samba 4.3.
The problem is related to a buggy and/or incomplete implementation of the "Backup Key" sub-protocol that older Samba versions have. You'll probably notice that Credential Manager was also unusable for the same reason.
Alternatively, use an older OS to manage the directory – e.g. Server 2003 can create user accounts without any problems, since it doesn't use the "Backup Key" protocol for anything yet. (I am not sure about Server 2008.)
Unfortunately, 4.3 is not yet packaged for Ubuntu – still pending in Debian's "unstable" too – however, it's actually easy to build Samba from source code.
As for the [Users] block, it simply defines a shared folder.
In smb.conf, every section except [global] corresponds to a share definition – "netlogon" and "sysvol" are special AD shares, and "Users" is probably a custom one.