Plex Docker rights

I would avoid using ACLs in OMV unless you know exactly what you are doing. Same thing with permissions manipulated in the OMV GUI.

It isn't complicated. The vast majority of Plex problems are directly related to not having the proper permissions set on the files and folders such that user the program runs as has the correct access permissions. Wrapping Plex in Docker adds another opportunity to get this wrong.

The user the docker container runs as must have sufficient permission to access the media files and folders, and the container config folder.

This means at least read permission on your media files, read and execute permission on your media folders, and read, write permission on the directory that the /config docker container path is bind mounted into.

Having the applicable media files and folders owned by root is not a good idea. You should have created a separate user for all this stuff.