Reviving an old question, but I’m struggling with this issue as well. Increasing the compression quality, lightmap resolution, indirect lighting quality, or indirect lighting smoothness has little effect. The only acceptable solution I’ve found so far is to fully disable lightmap compression (set World Settings->Lightmass Settings->Compress Lightmaps to false). Of course, this will consume a lot more memory (up to 4x more), but it does produce smooth, clean results. Are there any better solutions out there?