I noticed a strange problem with the Progressive Lightmapper when working on a map for our mobile game. It is a simple scene created from 3d tile pieces and some props (trees, rocks, etc.). Lightmapping was working fine, until at some point (after I had added more props) it stopped baking properly. “Preparing Bake…” progress bar is shown but it keeps restarting indefinitely. Strangely, when I switch away from Unity to another program (can be anything, like an Explorer window when Unity is still visible), I can see that the bake goes over the “preparing bake” and bakes normally. If I switch back to Unity when the bake is in progress, it will get restarted again. The same happens on three different computers, so it is not specific to the computer.
Enlighten bake finishes normally, but it is dog-slow and produces worse results, so I wouldn’t be using it anyway. There is nothing special about the scene, as I said this is a mobile game so very simple stuff really. I have grass shader that wiggles vertices of the grass meshes, which could in theory cause this, because the geometry keeps changing, but toggling “animated materials” on the scene view doesn’t help - and it did work at first until it just stopped working.
Any ideas? 
Lightmapping settings are as shown:
