Do you have a pihole setup and running inside your network?
No, I don't. Now i figured out how to access it: I have to use the IP adress instead of the hostname to access the Plex configuration. The funny thing is that I can reach my OMV installation by the hostname, ping on Hostname / FQDN works. DNS is configured properly.
Now I try to mount my media files. I have set host path as "/srv/...." and container path "/media" but media is empty. I have already given the dockeruser access to the folder.