rtmp stream not working

I'd like to use MS Teams to live broadcast our school lessons using RTMP. Unfortunately I'm not able to push RTMP video to your servers. I have correct encoder settings. See https://docs.microsoft.com/en-us/stream/live-encoder-configuration

I'm using both ffmpeg and OBS encoders. None of them are not able to push the video. Ffmpeg is able to connect, but it fails with error "RTMP_ReadPacket, failed to read RTMP packet header"

My ffmpeg command:

ffmpeg -f lavfi -i anullsrc=sample_rate=48000 -strict -2 -y -loglevel trace -f x11grab -s 1280x720 -framerate 30 -i :0.0 -threads 2 -vcodec libx264 -pix_fmt yuv420p -preset fast -acodec aac -strict -2 -f flv rtmp://xxxxxx.channel.media.azure.net:1935/live/xxxxxxxxxx

The user interface on microsoftstreams.com is saying:

"Ready to connect

Could you please help me?

Jan