I have this problem.
Hey jkaszewski -
As the error says, your UVs are overlapped in your lightmapping. Can you post a screenshot of your UVs (both channels) and we can help identify the problem areas of the unwrap.
-Eric Ketchum
Yesterday i import fbx files to my project and i make a lighting build and project work fine. Today i have this problem with ALL of my mesh have this error. I dont change anything or i dont know about a changes 
Hey jkaszewski -
Yes, your meshes are missing a lightmapped UV sets which will produce add artifacts if it doesn’t produce a black mesh on light build. Take a glance at this webpage for some help getting started. As to why it happened “all of a sudden,” did any of your lights change, if lights and meshes are moveable then lighting becomes dynamic and light maps are not used.
-Eric Ketchum