My problem was just the NextCloud configuration in the GUI.
There is no “NextCloud Office” app as you would expect and no “Office” in settings. I had to install “Collabora Online” app, which was then automagically renamed to “NextCloud Office”, only then “Office” showed up in settings.
But this is not documented - and makes no sense to me 
I set up NextCloud and NextCloud Office with Traefik and Let’s Encrypt in Docker Swarm.