Opera seems not to be able to play live streams on Youtube.
Cannot play live Youtube on other streams in Opera - no h264 support
The solution would be to install chromium-codecs-ffmpeg-extra.
Im my case I have Opera Version:77.0.4054.277: chromium-codecs-ffmpeg-extra_77
But where can I download this rpm file which is supported also by Mageia?
It works fine with the Chromium browser.