Thanks guys. I was playing on auto turn and pressing the end turn button solves the problem. However, I'd still like there to be a way that I can continue playing on auto turn with some cities idle (like in Civ 5, when you can set them to research etc.).
Having said that, I actually really like the fact that you can't move units on auto turn when you haven't assigned a task to a city. For me, this small change makes the difference between the game being playable and unplayable. Before this, I was always leaving cities to idle by mistake.