Cannot access Plex server

Hello,
I just installed docker and then Plex following this guide: OMV + Docker plugin media server (Plex, PlexPy, Ombi, Libresonic, NZBGet, ruTorrent, Sonarr, Radarr, Mylar, and more)
Network is set to host and the config directory point to the folder I have created as instructed in the tutorial. Also double checked PUID and GID for the dockeruser.
If I try to access Plex I am being able to log in but the webinterface keeps telling me that it is searching for server like forever with a spinning circle.
What caused this and how to fix it? Any help appreciated.

Quote from devnet

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.