I just updated my unity to 5.4.1. I can play normally in the engine itself, but when i build my game i get errors like these:
The type or namespace name `NetworkBehaviour' could not be found. Are you missing a using directive or an assembly reference?
The type or namespace nameUnityEngine'. Are you missing an assembly reference?
UI' does not exist in the namespace
The type or namespace name `IPointerUpHandler' could not be found. Are you missing a using directive or an assembly reference?
I used to have the same problem, I fixed it with
Edit: It is a well know Issue of Unity, if that doesn´t work, try deleting the
Library\UnityAssemblies folder. After restarting Unity, the folder will appear and probably will fix your problem.