Looking closer at the logs, I see the exact same error message that @tbd52942 reported a few months back here:
Apr 27, 2021 22:51:01.847 [5112] ERROR - [Transcoder] [Parsed_scale_qsv_1 @ 026b6400] Error synchronizing the operation: -21
Apr 27, 2021 22:51:01.848 [5112] ERROR - [Transcoder] Error while filtering: Unknown error occurred
Apr 27, 2021 22:51:01.848 [5112] ERROR - [Transcoder] Failed to inject frame into filter network: Unknown error occurred
Apr 27, 2021 22:51:01.849 [5112] ERROR - [Transcoder] Error while processing the decoded data for stream #0:0
Apr 27, 2021 22:51:01.942 [7388] DEBUG - Jobs: ‘C:\Program Files (x86)\Plex\Plex Media Server\Plex Transcoder.exe’ exit code for process 860 is 1 (failure)
Apr 27, 2021 22:51:01.943 [7712] DEBUG - Streaming Resource: Changing client to use software decoding
