I've been running plex from the app store of a server running TrueNas OS, which when I installed it, was running the linux fork, but nowadays seems to be the only version. I had been running a rolled back version of plex for a long time because when I tried to update it, my 'smart' TV would show this error when attempting to play some of my video files:

 >Playback Error
 >Conversion failed. A required codec could not be found or failed to install.

Unfortunately, recently I came back from work to one of these emails:

>Dear Plex User,
>We recently released Plex Media Server 1.43.3 and Plex Desktop 1.115.0
>to address a number of security issues. We recommend all server owners
>and Desktop users update to the latest version as soon as possible.
>CVEs have been requested and we'll share more details in our forums
>once they're published.
>If you're running Plex Media Server on a NAS device, the updated
>version may not be available in their package manager yet but you can
>install the package manually. See this [2]forum post for instructions
>on updating your server.
>Sincerely,
>Your friends at Plex

Womp, womp. My hand was forced, and I had to update. I was hoping that I held off long enough that the new one would have a fix for my issue. No such case, the problem was still happening, and on the exact same files I remember causing them last time. Turns out, it wasn't the actual Plex server software, the docker-ization packaged for TrueNas is misconfigured.

FIX:

Find wherever your codecs directory is, the apps page > plex > settings will show you where your container's configuration folder is.

Open the webpage system shell and navigate to:

(your PlexConfig path)/Library/Application Support/Plex Media Server/Codecs/

If you see something like:

EasyAudioEncoder-0dd4de86f2b9bb8b9b397a90-linux-x86_64.tmp

change the name to remove the .tmp at the end, videos work now!