Why is VS15 throwing errors for missing namespace 'UnityEngine' when it is already included

I've been using VS15 for a while now with Unity (5.3.3f1, now upgraded to 5.3.4f1 recently) and it was initially working fine.

However recently everytime I try to debug my program, I get between 18-25 errors regarding missing namespaces.

For example;

 "The type or namespace name 'EventSystems' does not exist in the namespace 'UnityEngine' (are you missing an assembly reference?)"

This particular error is in the AxisTouchButton.cs file (which I do not touch - not even use at the moment). The other errors are in Joystick.cs, TiltInput.cs and Touchpad.cs

This is a collaborative project, but to my knowledge no one on my team has touched that file either.

The even more wierder thing is, if I remove the namespace in one of the files....then re-add it back in. It will remove all the errors. But then when I try to debug again they all return.

Now, when I press "Run last working build" it will run fine. The even more weirder thing is that, this 'last build' is the build I had intended to run before the errors appeared. So for all intensive purposes it still runs fine despite these 18 errors. I just have to go through an extra dialog box in order to debug the program. But its very annoying (and concerning - as I fear it may produce additional issues down the line).

So if anyone is aware of the cause, and how to prevent/stop it, I'd greatly appreciate some support! :)