Same problem here, but with almost all videos. If you open a video in a fullscreen mode, after you close fullscreen the browser does not have any tabs or side panel or anything, only the current tab. By clicking escape button you can transform fullscreen opera back to tab view, but only as a window on the main screen, and it seems impossible to enter normal fullscreen browser without shutting it down first. Before the update to Opera 70 (which i did not ask for) everything worked fine.