Opening the Advanced_Lighting Map, the new default map with 4.5.0 and 4.5.1, the Skybox has 4 UV Channels but all of them have overlapping UVs.
You can Auto Generate UVs for the Skybox but it will overwrite the unwrapped Channel 1 that already exists. (This workaround has been posted on the Answerhub.)
1. Open a New Project with Starter Content
2. Open the Advanced Lighting example map.
3. Build Lighting
Error Skybox has 50% Overlapping UVs
No Error generated on Light Build
Head over to the existing AnswerHub thread and let us know what's up.