[FFmpeg-user] Trouble-shooting "The hardware encoder may be busy, or not supported"

[Media Mouth]

Trying to make use of '-vcodec h264_videotoolbox'.  I've used it on a lot of different machines, but am currently working on a system that's giving the error 'hardware encoder may be busy, or not supported'.

This may be entirely true.  It's an Avid with 3 virtual monitors, conneccted to JUMP App for remote operation, and also hooked up (I'm not entirely sure how) to "ClearView Flex" (software that creates a virtual Avid screening room) and this could well be the culprit, perhaps hogging GPU usage?

But just in case there's some way of testing / tweaking the ffmpeg CLI that in a way that makes use of one of those GPUs for transcoding from DNxHD/HR to mp4, I'd love to figure it out.

We're on an old MacOS (Mojave)
There seem to be 2 GPU cards, in slots 1 & 2, both AMD FirePro D500, 
(I can post specs from the Apple Menu > About this Mac > System Report... if that might helps)

Command that's failing:
	ffmpeg -i /path/to/source.mov \
	-vcodec h264_videotoolbox \
	-b:v 6000k -vf format=yuv420p \
	/path/to/dest.mp4

Output is...

toolbox -b:v 6000k -vf format=yuv420p /path/to/dest.mp4
ffmpeg version 4.2.2-tessus  https://evermeet.cx/ffmpeg/  Copyright (c) 2000-2019 the FFmpeg developers
  built with Apple clang version 11.0.0 (clang-1100.0.33.16)
  configuration: --cc=/usr/bin/clang --prefix=/opt/ffmpeg --extra-version=tessus --enable-avisynth --enable-fontconfig --enable-gpl --enable-libaom --enable-libass --enable-libbluray --enable-libdav1d --enable-libfreetype --enable-libgsm --enable-libmodplug --enable-libmp3lame --enable-libmysofa --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenh264 --enable-libopenjpeg --enable-libopus --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvmaf --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs --enable-libxvid --enable-libzimg --enable-libzmq --enable-libzvbi --enable-version3 --pkg-config-flags=--static --disable-ffplay
  libavutil      56. 31.100 / 56. 31.100
  libavcodec     58. 54.100 / 58. 54.100
  libavformat    58. 29.100 / 58. 29.100
  libavdevice    58.  8.100 / 58.  8.100
  libavfilter     7. 57.100 /  7. 57.100
  libswscale      5.  5.100 /  5.  5.100
  libswresample   3.  5.100 /  3.  5.100
  libpostproc    55.  5.100 / 55.  5.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '/path/to/source.mov':
  Metadata:
    major_brand     : qt  
    minor_version   : 537199360
    compatible_brands: qt  
    creation_time   : 2023-04-13T01:55:36.000000Z
    timecode        : 01:12:58:01
  Duration: 00:11:45.87, start: 0.000000, bitrate: 36139 kb/s
    Stream #0:0(eng): Video: dnxhd (DNXHD) (AVdn / 0x6E645641), yuv422p(tv, bt709), 1920x1080, 36139 kb/s, SAR 1:1 DAR 16:9, 23.98 fps, 23.98 tbr, 23976 tbn, 23976 tbc (default)
    Metadata:
      creation_time   : 2023-04-13T01:55:36.000000Z
      handler_name    : Apple Video Media Handler
      encoder         : Avid DNxHD Codec
    Stream #0:1(eng): Data: none (tmcd / 0x64636D74) (default)
    Metadata:
      creation_time   : 2023-04-13T01:57:23.000000Z
      handler_name    : Time Code Media Handler
      timecode        : 01:12:58:01
Stream mapping:
  Stream #0:0 -> #0:0 (dnxhd (native) -> h264 (h264_videotoolbox))
Press [q] to stop, [?] for help
AVDCreateGPUAccelerator: Error loading GPU renderer

[h264_videotoolbox @ 0x7fd232005600] Error: cannot create compression session: -12915
[h264_videotoolbox @ 0x7fd232005600] Try -allow_sw 1. The hardware encoder may be busy, or not supported.
Error initializing output stream 0:0 -- Error while opening encoder for output stream #0:0 - maybe incorrect parameters such as bit_rate, rate, width or height
Conversion failed!