[Samba] DNS not working, samba not listening on port 53

[Joachim Lindenberg]

I joined a DC, but the DC does not listen on port 53. When I do a  netstat
-a -n -p |grep 53, I can tell that systemd-resolv listens on 127.0.0.53:53,
but nobody is listening on 192.168.177.18:53 or 127.0.0.1:53 as I´d expect
(and is the case on my previous DCs except of course the IP address
differs).

Don´t know if that matters: I forgot to specify an upstream resolver during
join, but added that later on in smb.conf. 

The new DC is on ubuntu 18.04.2 and running Samba 4.10.5 of Louis.

Everything else I tried works: sysvol is available, RSAT tools work.

What can be the root cause?

Thanks, Joachim