Here's a few lines from the log when the errors first occur =
07:14:42.886: [ffmpeg/video] h264: sps_id 0 out of range
07:14:42.886: [ffmpeg/demuxer] avi: Failed to open codec in avformat_find_stream_info
07:14:42.887: [ffmpeg] NULL: sps_id 0 out of range
07:14:42.887: [ffmpeg/video] h264: sps_id 0 out of range
07:14:42.889: [ffmpeg/audio] mp3float: Header missing
07:14:42.892: (+) Video --vid=1 (h264 720x480 29.970fps)
07:14:42.892: Audio --aid=1 (mp3 2ch 48000Hz)
07:14:42.905: [ffmpeg/video] h264: sps_id 0 out of range
07:14:42.905: Could not open codec.
07:14:42.905: Decoder init failed for h264
07:14:43.318: [ffmpeg/video] h264_qsv: Error initializing an internal MFX session: unsupported (-3)
07:14:43.318: [ffmpeg/video] h264_qsv: Error initializing an MFX session
07:14:43.318: Error while decoding frame!
I am migrating from one PC to another. This happens on the new machine. All files transcode ok on the old machine.
The video in question plays fine in VLC on the new machine.