One can also put it like this: At least one vertex has to be a member of at least 4 different vertex groups that are linked to 4 bones. The original answer (by User 0000 here on the answer hub) was related (if I remember this correctly) to blender using float2 and float3 for describing vertices and their corresponding weights while other big packages only differ float1 and 4. And as the Unreal apex importer imports either float1 or 4 too it won’t be able to handle float 2 or 3, hence the weird error appears.