I just wanted to add a comment to this. Until recently I had FRAPS running all the time, just to give me my FPS count. I've started playing Marvel Heroes 2015 a few days ago and wanted to stream with OBS, but each time I started "preview" and then swapped to the game, the capture/stream froze... luckily I had some game crash pointing out fraps64.dll, so after quitting FRAPS streaming started working without any further hickups!
Basically, using any other screen overlaying while streaming, can result in some weird scenarios.