Huh, after deleting/reinstalling your Lightmass and restarting UE4, it worked.
EDIT:
After running a few successful tests on smaller maps, I tried to run a bake on a pretty big map using the command line. After the “Building BVH” bar gets to 100%, I get an error that says “invalid argument.” When I press OK on that, it gives me this: https://i.imgur.com/pwswHGe.png
After hitting “retry,” it fails and then gives me this output log: [2018.05.27-14.58.25:921][ 0]LogStreaming: Display: Took 0.310s to EndInitText - Pastebin.com