Seeing this error in the log
May 19, 2019 12:34:46.878 [0x7fcff0ff9700] ERROR - Failed to calculate available disk space for path '"/transcode/Transcode/Sessions/plex-transcode-e77e6c2a-6ef2-452a-924c-f3014201d5b2-4-74b8c2ca-e197-4919-a470-484336eae362"': 1 (Operation not permitted)
Can you look into permissions for /transcode/Transcode/ for the plex user account ?
Also transcode temp directory must not be a network share
See this forum post which was for a similar issue Conversion failed. The transcoder crashed or failed to start up. - #8 by sa2000
Only difference is that codec directory for this version of Plex Media Server for your platform is
/config/Library/Application\ Support/Plex\ Media\ Server/Codecs/392df4f-2183-linux-x86_64/
