SOLUTION: Conversion failed. The transcoder exited due to an error

The point is that it must not be a network share and also it needs to have execute permissions for the codecs binaries to run.

There is a section concerning the /config path in the Plex Docker readme here https://github.com/plexinc/pms-docker/blob/master/README.md

  • -v <path/to/plex/database>:/config The path where you wish Plex Media Server to store its configuration data. This database can grow to be quite large depending on the size of your media collection. This is usually a few GB but for large libraries or libraries where index files are generated, this can easily hit the 100s of GBs. If you have an existing database directory see the section below on the directory setup. Note : the underlying filesystem needs to support file locking. This is known to not be default enabled on remote filesystems like NFS, SMB, and many many others. The 9PFS filesystem used by FreeNAS Corral is known to work but the vast majority will result in database corruption. Use a network share at your own risk.

Also the path to the transcode directory must not be on a network share

-v <path/to/transcode/temp>:/transcode The path where you would like Plex Media Server to store its transcoder temp files. If not provided, the storage space within the container will be used. Expect sizes in the 10s of GB

See forum threads concerning this

Other possibility is that we are unzipping to a temp file and the temp directory does not allow the execute permission - where is /tmp pointing to?

For mounted volumes there would be an entry in /etc/fstab which would be where the exec / noexec be - but databases and transcoder directory must not be on a networked share