I have had all the same issues you describe. Turning off hardware acceleration for the popout may help. See these posts:
Posts made by taiichi-090
RE: Video pop-out scaling problems and crash with YT clipsOpera for Windows
RE: HTML5 video issuesOpera for Windows
You could try turning off hardware acceleration for the pop-out video, which seems to be in issue in the new version. (Actually, it's an issue for people's video hardware... but the distinction is academic here.)
Go to "opera://flags" and search for "Hardware accelerated detachable video" (or go right to "opera://flags/#media-detachable-view-hwaccelerated"). Switch it to Disabled if it's not. This may help for the time being.