It is the same issue as was seen when windows 10 users applied windows update July 10, 2018—KB4338819 (OS Build 17134.165) - Microsoft Support
Jul 15, 2018 20:26:36.969 [6648] ERROR - [Transcoder] [stream_segment,ssegment @ 02d7a800] Failed to open segment list 'http://127.0.0.1:32400/video/:/transcode/session/4o6m2eu7qh6zs4bgkpbm9ctb/211ae52b-ec7b-426a-b906-02803406c7ea/seglist'
Jul 15, 2018 20:26:36.996 [11084] ERROR - [Transcoder] av_interleaved_write_frame(): Unknown error
Jul 15, 2018 20:26:42.287 [10696] DEBUG - Streaming Resource: Terminated session 0x4f7f028:xhc2tyg7i1q86zi3et40gqzz with reason Conversion failed. The transcoder exited due to an error.
Could you list the windows update history and see what was applied recently for your windows 8.1 OS
May be there was a windows 8.1 update with same issue.
The windows 10 issue with the update was fixed by a further update July 16, 2018—KB4345421 (OS Build 17134.167) - Microsoft Support
