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First off, @henriquevigil Thanks, that fixed it for me. Then I'd like to point out that this is not the same all the time. I mean, I saw your and decided to try it on a different machine (same OS version, same Opera version, same browser extensions) and it was working fine. I set the player on theatre mode, then turned fullscreen, all was good.
After update 95 something happened with YouTube on Opera. I can do most of basic stuff without problem, but when I try to go fullscreen mode, I can hear video's sound going, but the screen is completely black. I have tried watching fullscreen on other sites and it worked, also tried YouTube fullscreen mode on a different browser (Mozilla) and it works perfectly fine there. I am not sure what is causing this issue because yesterday it was working perfectly fine and I have not changed anything in Opera's settings.
Below are 2 images combined, the upper one shows what I see on standard mode, the lower one shows what I see on fullscreen.
