Ok, I finally figured it out. There is a new option in the OBJ import settings. It must have come in with one of the LW beta updates (EDIT or it was there all the time and I changed it unconsciously /EDIT). Because at the moment all my LW updates are pointing to the same config file this was activated in all versions. It's the "OBJ ZBrush Mode". After switching it on, I get the parts again in all LW beta versions.

But now I have to face, that I don't really understand OBJ. What is a part, what is a surface, what is a group (Poser) etc? What happens when exchanging between Poser, LW, Blender, Wings, 3DCoat etc?

Do you know a ressource where this is explained in more detail? It would help to do less guesswork? I'll also google to find something.