LG webOS Plex: "Playback Error" when a subtitle is selected

Plex Players
1

Server Version#: 1.43.4.10903
Player Version#: (Client) 5.94.3, (Platform) 10.3.1

Symptom

TV Shows shows with subtitles turned on show “Playback Error - An unexpected playback error occurred” almost immediately on my LG TV. The same files play fine on iOS, iPad and a Samsung Frame (with ASS, PGS and SRT subtitles).

Environment

  • TV: LG OLED83C4AUA, webOS 10.3.1, Plex for LG 5.94.3 (reinstalling the app did not help)
  • Server: Plex Media Server on Windows, NVIDIA hardware transcoding (nvdec/nvenc) working

Findings (from server logs)

  • With subtitles set to None, the file plays (HLS direct stream, audio transcoded from Opus). Opus and 10-bit HEVC are not the problem.
  • With any subtitle selected, the LG app sends subtitles=burn and playback fails. Tested and failing: embedded ASS, embedded SRT (converted with ffmpeg), sidecar .srt, at 1080p and at 720p/4 Mbps.
  • A re-encode without a subtitle (720p, subtitles None) works, so re-encoding itself is fine.
  • Failure signature: the TV’s requests for /video/:/transcode/universal/session/<id>/base/index.m3u8 reach the server as ET /video/... (first byte missing), Plex logs “Error parsing HTTP request”, returns 400 (11 times in ~70 ms), the transcoder never starts, and the client reports state=stopped.

Not established

Why the TV’s player produces malformed requests only in the burn-in case. Cause is presumed to be a client-side bug in Plex for LG or webOS, but this is inference.

2

Do you possibly have transcoding disabled on the server?