Hello from the future. For people having this problem, here’s the key takeaway:
The name that you give the armature, in blender, matters. That will be the name of the root bone in Unreal Engine.
Whenever you export an armature from blender, to fbx, for use in UE, rename the armature to "root".
If you have two armatures in the same file, that means you need to rename both of them, one at a time, before exporting the armature, because blender will not let you have two objects with the same name.
TLDR: If you have two armatures, which you think are supposed to be identical, but UE is throwing errors, check the name of the root bone in UE, which is the same as the name of the armature object in blender.