Failing to solve the problem, I re-installed everything all over again.
This time, the first thing I tried to configure was smb: well, it gave me the same problem.
The only thing I was deviating from the standard procedure is the manual ext4 formatting.
Why was I doing it? Because for some reason the formatting via web interface was very slow (hours for a 4Tb drive).
So I was resetting disks, creating and configuring Luks via UI but creating the filesystem by hand with mkfs.ext4.
The partition is correctly read in the "File system" menu but I couldn't configure Samba.
Creating the partition with the web interface works instead.
I point out that users, permissions, shares, are exactly the same, also checked in the config.xml file.
There is definitely some part that I didn't catch but I didn't understand which one.