Since yesterday, we’ve gotten a number of incoming reports that selected YouTube videos no longer play in Opera. We’ve investigated this further and it seems to be a recently introduced codec selection issue on YouTube’s side. We’ve contacted YouTube about this: their friendly engineers are on the case, and will release a fix for the problem soon.
In the meantime, we’ve released a browser.js patch that works around the issue by forcing YouTube to always use the Flash player for playing videos. You should get the file 2-3 minutes after restarting Opera. We will remove this patch once YouTube’s fix is released.
Let us know if this doesn’t work for you.