Are your pieces cubes that you are just snapping together? Like, when you put one chunk together with another, do you have two faces meeting in between them?
If that's what you are doing, then you just need to delete those inner faces that are touching. Your light map is baking around the corner and giving you what looks like a seam.
It works fine with lightmass off, because it's more than likely just doing a vertex light and not baking the light maps.
Hope ya get it workin.