tutorial to plex install:
https://www.elefacts.de/test-1…en_unter_openmediavault_5
tutorial to plex update:
https://www.elefacts.de/test-1…er_openmediavault_updaten
tutorial to docker install:
https://www.elefacts.de/test-1…eue_nas_kopieren_mit_omv5
If /usr/lib/plexmediaserver is really the leftover of the previous docker install, you can delete it, but from the name it sounds like a native install.
I think, this is the installation with the tutorial.
Is it possible, that docker is not right installed?