Nah it was a bug with Nextcloud Docker 20.0.6… 20.0.7 took a while to be ported to docked. All good now.
The only thing I cannot manage to get working, neither with your docker installation or with my zip installation on a different server is the coturn server… I always get the “no ICE candidates were found”… I noticed on your description, at the Nextcloud Talk point, it says “use cloud.domain.name:3489” and then “forward TCP and/or UDP port 3478”. Looking at the docker-compose.yml I can imagine the right port is 3478… I tried both and it doesn’t work either way.
EDIT: Turn Server (Coturn): Einrichtung funktioniert nicht - #15 by FatBastard