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 ntsc -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, 29.97 fps, 29.97 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 ?