The last time such errors were widespread was when Microsoft released a faulty windows 10 update on 10th July 2018 which they subsequently corrected in the July 16th Update - but the impact remained widespread for months after.
The problem manifests itself in loopback request failures and logging of errors like these
Nov 26, 2018 19:08:37.660 [10512] ERROR - [Transcoder] [stream_segment,ssegment @ 04ccb080] Failed to open segment list 'http://127.0.0.1:32400/video/:/transcode/session/ba2d6b30-8b9a-48aa-952b-b166e3228b76/98fcd8a7-3bc0-4491-a091-a6003569532b/seglist'
Nov 26, 2018 19:08:37.660 [8656] ERROR - [Transcoder] av_interleaved_write_frame(): Unknown error
Nov 26, 2018 19:08:37.660 [10512] ERROR - [Transcoder] Error writing trailer of media-%05d.ts: Invalid argument
It is very possible that there has been a further update to windows that brought this problem on. So first advice is to check for windows updates - not just critical updates
If that does not resolve, then would need to see if we can capture packets between the transcoder and plex media server that come in through 127.0.0.1
Also you had debug logging disabled on the server - you would need to enable it and for transcoder issues would also need verbose logging - but that would mean you need to capture logs immediately after the failure as each log file may fill up in 5 minutes with verbose logging
To capture traffic through 127,0,0.1 there is a very easy to use program - RawCap.exe which can be used.
See my post here on the subject SOLUTION: Conversion failed. The transcoder exited due to an error - #307 by sa2000
