Any update?
I've been having this problem for a few months now.
It only shows up in Youtube.
I disabled hardware acceleration and it works fine.

But, without hardware acceleration the whole browser is stuttery, and I don't like it.
The same thing happens in Chrome btw.

Fixed here!

Well>> They have just updated Opera to version 21, but didn't fix this problem!

It is not an Opera problem.

Well, in User-Agent Switcher, if you select Opera 10, it still works. So my guess is google doesn't recognize Opera as Opera. It identifies it as Internet Explorer 6, or 7, or 8...

They have just updated Opera to version 21, but didn't fix this problem!

Same here. Clearing all cookies did'n work, but the add-on did. Hopefully they'll fix it soon.