I'v tried 0.33.(1,2,3,4) and 0.34.0 libffmpg that I'v got from here: https://github.com/iteufel/nwjs-ffmpeg-prebuilt/releases/
(the screen was for reference only). Youtube crashes whenever I copy libffmpeg to Opera directory. If the ffmpeg is deleted videos are not working but the site does not crush.

@bombacha in mine case the plugins are cousing Opera to crash I got similair screen to that https://forums.opera.com/topic/25021/opera-51-0-settings-page-crashed
I use Opera 64 bit version 56.0.3051.104. on Debian Stretch. I'v tried 4 different revisions of libffmpeg.so. I eaven tried the 32 bit, the Problem is always there.

On 54.0.2952.64 H.264 does not work.
that's it. I'm tired of searching solution every few months.