Alessio, I made a few suggestions above for additional info that could be added.
Also, I went back and checked ClearOS to see what info they had to compare and I think that after initial setup I can't see that anymore. I'd need to recreate the vm from scratch and try again. When I get time I will do this to see what helpful info there is.
I can tell you that I've now tried at least 6 fresh installs of NS and I can't get file shares to be accessible from Windows or Mac - and/or the web ui crashes and I can't make progress.
Zentyal: Fresh install, setup "Domain (Samba DC)", create a user and a group with that user. File sharing ("Flexshare setup as Windows File Sharing") works as long as an ACL is created for the shared folder.
ClearOS: Fresh install, setup "Windows Networking (Samba DC)", create a user and a group with that user. File sharing works without doing anything else.
Nethserver: Still trying...