Did you check in Opera Developer to see if the issue is already fixed? Does Opera Stable still work fine?
Did it just start with Opera 64.3417.8? In https://blogs.opera.com/desktop/changelog-for-64/#b3417.8, I see Opera was updated to use Chromium 77.0.3865.42. I don't see any fixes/changes for video handling. There might be some in the Chromium Changes since the last Chromium version Opera had though.
What OS are you using? If Windows, is it an "N" or Enterprise version? If an "N" version, maybe the Windows Media Pack got uninstalled somehow.
Does https://www.youtube.com/html5 say that you're awesome?
Does the video at https://bitmovin.com/demos/drm work? What does it say for MSE and EME below the video?
Does Windows Media Player still play videos?
If you goto the URL opera://settings/system, disable hardware acceleration and restart, does it work then?
Did you try with a standalone installation of Opera Beta (don't use Opera Sync in it and don't install any extensions) to see if it's just your profile that's messed up?