Unity 5.1.1f1 hangs on repacking sprite atlases

So, this is what happens:

I change some images or add new ones to Unity or directly into an Asset subfolder and upon build/play Unity hangs on repacking sprite atlases. This happens whether I play in the Editor or I try to build for iOS. As you can see, it is not responding and I have to force quit it. Restarting Unity does not help, removing Library and Temp folders from the project folder does not help, restarting my Mac does not help. It happens (or at least it appears to happen) randomly, no matter which image I add or change.

Is there any way to find out with which sprite atlas Unity has issues or with which images?

We are making a 2D game with a lot of images (thus a lot of atlases). Unity version is as mentioned in the title, 5.1.1f1.