I have confirmed that Opera 31.0.1889.174 crashes when a video starts in a different discussion to a guy with Windows 10 and the discussion was immediately closed with an explanation that Opera 31 is no longer in development. I have Windows 8.1 64.
And does it happens when you are using Whatsapp Web?
It happens on any web with a flash video, but not always so it's difficult to replicate. HTML5 videos seem to play fine. I don't experience any problems on Whatsapp Web.