@aroton Hello, i've read your post, and let me tell you that i've been experiencing the same issues that you're describing, specially the video related ones.
3 days ago i made a post here about getting black screens when browsing Instagram video stories or just scrolling down in the site and when it happened if a was also watching a stream on Twitch for example, it also stopped working, all video playback things would crash at the same time and nobody said anything, not even a single comment.
Today for example i was also scrolling down on Facebook posts and when i came to see a post which contained a video, i inmediately got a black screen as soon as it showed and had to restart the browser to solve it.
Ive been getting those video related crashes very often and also tested on another completely different PC than my main one and i got the same issues on that one as well, both computers are completely up to date on windows update check, dont have any third party extensions and on the second one was even a fresh Vivaldi install, so i highly doubt that the cause are drivers, updates and things like that as they always say.
Like you, i've also been intensively browsing on other major browsers such as Opera GX, Firefox and Chrome, and on them all of those video related issues never show up, only on Vivaldi and, fair to say, only since version 4.1.
I never got those issues on version 4.0 or older, version 4.1 just seems to be full of bugs and problems, one of the things i use and like the most about Vivaldi is spatial navigation and it got completely broken on 4.1