2 days I was stuck with the fact that my 3ds Max exported bones as nulls, because they were deforming dummies. The “convert deforming dummies to bones” option didn’t work out upon export. I simply imported the default .fbx of 3rd person template skeleton from UE4 4.9 and exported it back to UE4, and it wouldn’t work.
The solution is that one has to select all the bones in 3DS Max, go to animation tab -> Bone Tools -> Object Properties rollout -> Bone On.
After that, 3DS exports deforming dummies as bones.
Just wanted to share this, so some other unfortunate human doesn’t have to be stuck with this for 2 days. I do not know if this is the right place to do so.