Hi @initeasyy, the "profile 11 and 12 is vp8 or vp9, so you won't get hardware decoding for it" - the source link.
You can try to work around of this problem, by forcing the use of the H264 codec for the video playback: install one of the following extensions: the "Extension source viewer", or the "Download Chrome Extension", and then install the "h264ify" extension, and see if this helps.

Did not work.

Oops, i messed up the formatting on that one. it says "[0706/141025:ERROR:gpuvideodecode_accelerator.cc(362)] : HW video decode not available for profile 11" a lot in there. but everything else looks normal

Check if opera://gpu says something about errors or issues.

Not sure what i'm looking for but it says

[0706/141025:ERROR:gpu_video_decode_accelerator.cc(362)] : HW video decode not available for profile 11
a lot in the "Log Messages" area.
Everything else seems to look normal to me.

It's the most recent one?

I belive so.

What OS and what video card? Does the crash happens only on Youtube or also in pther sites with html5 videos?

Windows 10 64bit.
GTX 660 with driver version 368.39.
I don't know if it's HTML5, but vimeo did the same thing.
Just tried full screen with Google chrome, and it acts as normal.

I'm now on opera 38.0.2220.41, and the problem still persists.

37.0 is a bit outdated now. Could you please update to 38.0.2220.41 and recheck if this still happens? Thanks.

I don't know if i'm missing something, but when i go to the opera://about/ page, it says "37.0.2178.41 - Opera is up to date"

Opera 37.0 crashes when i try to fullscreen youtube.
When hardware acceleration is off, this never happens.
But i need it on for some things, and for 360 video to work.
And i am using the HTML5 youtube player.