Hey, got a good news for you all. If you enable flag opera://flags/#dynamic-demuxer-selection issue should be resolved. It's still disabled by default in 54.0.2952.71 and will be enabled be default when 55 goes stable but you should be safe to turn it on now.
Same problem here for weeks! I was finally able to solve it with this flag change. Thank you!!
Meanwhile I was using either my phone or Vivaldi to playback the videos.