I'm having a very similar problem, but it only occurs on certain sites. For example, I try to play a video on the CNN site, and all that happens is a square box rolls around, no video. If I play my local TV station's weather report, it works fine. I don't have the problem when using Chrome.
Version: 40.0.2308.90 - Opera is up to date
Update stream: Stable
System: Windows 10 64-bit (WoW64)