Transcoder not working - can't find any erroors. Help!

[kepesk_gmail.com]

Server Version#: 1.13.8.5395
Player Version#: N/A

The Problem:
My Plex Media Server (running on Ubuntu Linux) has not been transcoding DVR recordings successfully for the past couple months. Everything appears to run okay (and commercials are deleted properly), but the file size is not reduced, indication to me that the transcoder has not output a compressed file.

What I have tried:
I looked in the Plex transcoder log but could not find any error messages or other indication of a failure.
I have tried hardware and non-hardware transcoding. It makes no difference.
I have tried changing the temporary directory, no effect.
I have double checked temporary directory permissions. They’re fine.
I have tried running the transcoder from the command line to create a sample file using the instructions at https://support.plex.tv/articles/201035968-generating-sample-files-from-media/ and it completes without error messages.

I’m completely stuck. I don’t know where to go from here. Could someone point me in the right direction?

Thanks!

[kepesk_gmail.com]

I’m still stuck with this problem, but I think I finally found a relevant error message. I’m getting this when the transcoder starts:

Nov 09, 2018 17:18:48.111 [0x7fa3e3ffd700] DEBUG - Job running: EAE_ROOT='/tmp/pms-bb0a857d-63f3-4e9a-8e44-59dc78d9870d/EasyAudioEncoder' FFMPEG_EXTERNAL_LIBS='/var/lib/plexmediaserver/Library/Application\ Support/Plex\ Media\ Server/Codecs/531e313-1328-linux-ubuntu-x86_64/' X_PLEX_TOKEN='xxxxxxxxxxxxxxxxxxxx' '/usr/lib/plexmediaserver/Plex Transcoder' '-y' '-safe' '0' '-skip_before_video_key' '1' '-skip_before_inpoint' '1' '-offset_inout' '1' '-i' '/media/disk-5/.grab/1d64d4de7a19a1b1496a66ef84e40c95d4471791/segment-list.txt' '-map' '0' '-c' 'copy' '-loglevel' 'quiet' '-loglevel_plex' 'error' '/media/disk-5/.grab/1d64d4de7a19a1b1496a66ef84e40c95d4471791/The Ellen DeGeneres Show (2003) - S16E48 - Judge Judy Sheindlin-comskipped.ts'
Nov 09, 2018 17:18:48.116 [0x7fa3e3ffd700] DEBUG - Jobs: Starting child process with pid 2025
Nov 09, 2018 17:18:48.559 [0x7fa3e4fff700] ERROR - [Transcoder] [h264 @ 0x1abe240] co located POCs unavailable

Anyone have any idea what that last line means?