A problem has been detected with a core component of Plex Media Server

[tom80H]

From what I could find, there seems to be an issue with certain container tags causing this kind of trouble.
There’s a post with a temporary fix by switching to a different tag:

Omv plex server error starting framework

Hi, So i have been having the same problem i have posted the question to the OMV forum hopefully someone is helpful and has an answer but a short term fix i found based on the answeer from someone on here is installing the “bionic” version of Plex which by the looks of it is the same and the “Latest” version just change the install line from linuxserver/plex:latest to linuxserver/plex:bionic and then hit deploy