First, I assume you're editing a mesh, not creating a new model mesh, correct?
When importing, the "Insert extra triangles to preserve morph targets" should be checked. This seems counter-intuitive since the general idea is to *not* modify the structure of the mesh, but this is indeed supposed to be checked.
When exporting, the "Use existing skeleton' option should be used. Select the original .l3d file from which the file was imported.
See if doing those two things works.
-ego533