Hi Gabriel,
thank you for your response. I’ll search more about the limitations of Samba as a DC ASAP. In my previous test environments they didn’t hit me, but it may be just me.
Meanwhile, I’ve repeated the whole test on my home PC, which has a very similar setup.
Host runs Ubuntu 15.04, IP address is 192.168.1.2, netmask is 24, gateway is 192.168.1.1, under a home router. VirtualBox is 4.3.26, and, repeating the steps in my previous message in the guest, using 192.168.1.20 for NS and 192.168.1.21 for the DC, the DC has been provisioned without errors!
Having checksummed the download on both my home PC and my office PC, and not willing to accept that the difference may be due to the host OS or VirtualBox versions 
I guess we can blame the network setup at office?
There we already have production a Windows Server 2003 domain with a primary DC (which is of course a DNS server, and has DHCP enabled) and two secondary DCs, the whole network is behind a firewall, but my PC is allowed to pass through for outgoing traffic, just like at home, and there is a running Zentyal 4.2 test server acting as PDC for a test domain (which is a DNS server, and has DHCP disabled).