I had this problem as well. I became so frustrated trying to fix the problem that I almost deleted Opera from my computer. I fixed the problem purely by accident (no idea why it worked.) This is what I did: In Opera, go to Preferences, Basic. Under Block Ads, uncheck the EastList and EasyPrivacy boxes. Restart Opera.
All is now working with Opera video playback for me. I am no computer expert. This is definitely some major bug with Opera. I hope that they fix it and hope that this fixes your problem as well. Good Luck!