I deleted the old container and pulled the new image. I also tested it with new volumes (-v test:/data) with the following command:
docker run -d -p 4443:4443 -p 443:443 -p 80:80 -v ncdata:/data --name nextcloudplus ownyourbits/nextcloudplus-armhf 192.168.0.222
It gets always stuck in the situation mentioned in my previous post, which therefore does not progress on the activation web interface.
I luckily found an old image because Docker has also updated the systemd service config to use the default docker directory again, so I was able to use the Docker image from February again.
With the new image I could not get it up and running.
(on another thread somebody has written about the same issue:
RPi 3B+ install issue: "...no commands defined in the "config:system" namespace")
I have tested the x86 image on my laptop and it is working, only the image for the arm architecture is not working for me on the rpi3.
Thanks for your work!!