Ok, so I deleted the local content, reinstalled/ re-downloaded the game and guess what....IT STILL DOESN'T WORK!
However, I'm now able to JOIN an online campaign. Although sometimes I get disconnected with an error 'could not authenticate user with Steam'. So basically, I can JOIN games (the passing) but NOT HOST them or even play it in single player.
Really dunno why this is happening. All my games are original, etc.