I've read just the beginning of the post, then I've had my usual issue with the monitors, then I was pissed off and I've wrote a post here (still no help... ☹️) and then I was working all the day.
Anyway, back to the subject: I've updated the system yesterday in the morning, then I've started all the applications that I use normally and in Vivaldi the "Whatsapp web" page wasn't working. It was displaying only the black icon.
So, I went in the directory where Vivaldi is installed /usr/share/vivaldi-stable/vivaldi, and there I've run the command ./update-ffmpeg.h.
Restarted the system and Vivaldi was working as usual.
Note: I don't use 2 versions of Vivaldi, just the stable one. But, I suppose that the same script is also in the snapshot. Hence, a small script that runs the command before launching a version or the other of Vivaldi could temporary solve the problem.