[5.2.1p3] Mono.Cecil.AssemblyResolutionException UnityEngine.UI.dll

Hi,

I am getting this error on Unity 5.2.1p3. Basically, Unity is failing to resolve UnityEngine.UI.

Any idea what could be causing this?

Thanks.

[5.2.1p3] Mono.Cecil.AssemblyResolutionException UnityEngine.UI.dll