I don't know if anyone still has this issue, but I found a solution. Just highlight your Opera GX, press F11 to go into full screen, open YouTube, select a video, then set the video to full screen and then back to normal mode. That should fix it, if not; you probably already had a video in full-screen when Opera forcefully closed, causing it to bug, just go to the tab of the last video you were watching and change it from full screen to normal mode, that should also fix it.