The only thing that comes to mind is it’s typical to have errors when importing a skeletal figure using a static mesh with in the hierarchy. There are a few things that goes bump in the night as UE4 does not even respect unique constraint requirements.
Best practice though is to import, in this case, the SK mesh and the SM mesh as separate elements and then add them as components as say part of a character BP.
Think of it as compositing your character where you can add detail to the base with out being locked into a single design where detail is baked into the mesh.