"Conversion failed. The transcoder failed to start up." message when trying to play videos with external subtitles

This is where I think the transcoder fails

Oct 26, 2020 10:49:42.379 [1884] INFO - [Transcode] [FFMPEG] - MFT name: 'Microsoft AAC Audio Encoder MFT'
Oct 26, 2020 10:49:42.387 [1884] INFO - [Transcode] [FFMPEG] - MFT name: 'Microsoft AAC Audio Encoder MFT'
Oct 26, 2020 10:49:42.525 [10884] ERROR - Couldn't find the file to stream: mycloudpr2100\TVShows\Alfred Hitchcock Presents\Season 1\S01E03.en.srt
Oct 26, 2020 10:49:42.526 [1884] ERROR - [Transcode] Error configuring transcoder: TPU: Failed to download sub-stream to temporary file
Oct 26, 2020 10:49:43.087 [3464] WARN - [Transcode] Denying access to transcode of key /library/metadata/23659 due to terminated session

Note the missing \\ in Oct 26, 2020 10:49:42.525 [10884] ERROR - Couldn't find the file to stream: mycloudpr2100\TVShows\Alfred Hitchcock Presents\Season 1\S01E03.en.srt

The right path is \\mycloudpr2100\TVShows\Alfred Hitchcock Presents\Season 1\S01E03.en.srt

I could be wrong but it looks like it’s a combination of the following

  • The media to be played has external subtitles
  • The media needs to be transcoded
  • The path to the subtitles is a UNC path.