Hi Gabriel,
I’ve restarted everything from scratch, following the steps in the post you’ve linked. Same result unfortunately. Just to be verbose:
- New virtual machine in VirtualBox 5.0.26, Ubuntu 14.04 host with IP address 10.0.0.91, LAN 1 as bridged adapter with promiscuous mode set to Allow All.
- Installed Nethserver 7rc1 using all defaults but Rome timezone, Italian keyboard, and static IP address 10.0.0.17, netmask 24, gateway 10.0.0.254, hostname set to ntest.nalma.loc (edit: .com was a typo), set a non-default root password, no additional users.
- Logged in in https://10.0.0.17:980 as root from my PC, applied updates in Software center. For the record, Software center took a long time to display packages, with a page reload helped in between.
- Edited contact information and self-signed certificate (relogged in after that).
- Installed Samba AD module, and only that.
- Reboot.
- Tried to configured the Samba AD module specifying 10.0.0.13 (free) as DC IP address, checked to create a bridged interface.
- Failed with the error already posted after several minutes.
- Without closing the page, with the same settings I’ve tried another go, same result after 15 minutes.
In past tests, the “factory reset” procedure didn’t help: same result over and over.
I’m starting to feel a little incompetent about that! Please let me know what do you think.
As a side note, I still don’t get why Samba has to be run in a container. Looks very not KISS to me. As said, starting a PDC in vanilla Ubuntu 16.04 or in Zentyal 4.2 takes literally a few minutes (tested joining a Windows XP client), while here I’ve watched one and half episode of Lost (no pun intended) while trying 
The motivation published by Microsoft, recommending that the DC is on a separate host, seems a bit thin to me: even they didn’t follow that in Windows SBS. Sorry if this may sound rude, it isn’t meant to, I’m just a little frustrated, as I’ve not yet started to try anything! 
Thank you for your help,
Salvo