Youtube – Cannot upload a WebM file containing OPUS audio and a still image as VP9 video to YouTube

I cannot upload a WebM file containing OPUS audio and a still image (a png file) as VP9 video to YouTube, YouTube says :

The video has failed to process. Please make sure you are uploading a supported file type.

This Google page says WebM is supported.

I used ffmpeg to create the WebM file with the following line :

$ ffmpeg -i myOriginialPureAudioFile-CUT.opus -i myAudioFile.png -ac 2 -af aresample=48k -b:a 128k -r pal -s 640x360 myAudioFile.webm

The resulting WebM file looks like this :

$ ffprobe myAudioFile.webm
Input #0, matroska,webm, from 'myAudioFile.webm':
  Metadata:
    ENCODER         : Lavf57.83.100
  Duration: 00:06:00.01, start: -0.007000, bitrate: 104 kb/s
    Stream #0:0: Video: vp9 (Profile 0), yuv420p(tv, progressive), 640x360, SAR 1:1 DAR 16:9, 25 fps, 25 tbr, 1k tbn, 1k tbc (default)
    Metadata:
      ENCODER         : Lavc57.107.100 libvpx-vp9
      DURATION        : 00:00:00.040000000
    Stream #0:1(eng): Audio: opus, 48000 Hz, stereo, fltp (default)
    Metadata:
      title           : unnamed
      ENCODER         : Lavc57.107.100 libopus
      DURATION        : 00:06:00.008000000

I tried different standard frame rates, bit rates, resolutions and audio sampling frequencies for half of my weekend, nothing seems to satisfy YouTube, can you please help me ?