Lightmass issues - Please help

Hi everybody, I am putting together a scene in UE 4.24.3. Most of the geometry is coming from MAX using Datasmith. On a couple of objects I am getting a strange set of random bright squares. I have spent hours hanging Lightmass settings but they keep showing. Can anyone help?
Just a note. The object has been unwrapped properly and the materials do not have any textures. They are very simple shaders.
Thank you in advance.