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Hi folks - this fix worked for me, though I would recommend you timeshift before you makes any changes to your system.
http://ubuntuhandbook.org/index.php/2020/06/install-chromium-via-deb-ubuntu-20-04/
In short, this installs Chromium (which is normally now unavailable): re-installation of this browser (and its codecs) fixes the video playback issue.
Hope this helps.
I understand the imagery, but don't like it. But this is because I don't like Greek-style symbolism is Christian art (Zeus etc).
No video playback for me either - with or without extensions.
Youtube, Whatsapp, Amazon product videos - the list goes on!
This issue has been going on for quite some time now, which is a surprise. Because of this I am having to use different browsers; where video playback does work - but this is a shame as I prefer Opera 
Version:69.0.3686.77
Opera is up to date
Update stream:Stable
System:Linux Mint 20 (x86_64; X-Cinnamon)