I had the same error message as the op and I followed your instructions which were really helpful but not completely satisfying. The textures/uv-/lightmap of my mesh do look better now by increasing the lightmap resolution which now is at 4096 but there are still some artifacts showing up. Also there were added some new ones.
I don’t think that still increasing the lightmap resolution to fix this artifacts will be performant. What else can I do to get a better looking lightmap?