Unity3D: kCGErrorInvalidConnection

If your Unity3D editor starts showing a blank, grey screen, check your Editor.log.

If you see this error:

kCGErrorInvalidConnection: CGSGetWindowBounds: Invalid connection

we found that the fix is to change your resolution to something lower, open the Unity3D project, then change your resolution back.

Update:Hmm, fix was only temporary, the problem persists.