Like others, in scenes with large terrains, we were hopelessly stuck during lightmap bakes at 5/11 Clustering. We have Precomputed Realtime GI turned OFF, so I ignored it for a long time. In desperation, though, I turned it back on, lowered its resolution down to 0.01 (from its default of 1), then turned it back off again. And voila - we are no longer stuck. We are now able to bake successfully. Therefore, it seems like something in the bake process is not paying attention to the fact that Precomputed Realtime GI is turned off, and is still trying to do some work based on its resolution parameter?