Fresh install of NCP in docker but nextcloud check error

I installed NCP with the command

docker run -d -p 4443:4443 -p 443:443 -p 80:80 -v ncdata:/data --name nextcloudpi ownyourbits/nextcloudpi-x86 $DOMAIN

on my system (fresh Ubuntu 18.04 LTS / Docker 18.06.1-ce) and opened the start page 192.168.x.x, logged in as ncp and activated the system. Unfortunately the dashboard will than show up under System Info:
Nextcloud check: error
I have no idea where to look for the cause and how to get ahead.

Strange for me is there is under /var/snap/docker/common/var-lib-docker/…/ncdata/
a subdirectory …/_data — shouldn’t this be …/data?

How can I solve the problem? Any help is welcome!