Blender For Unreal Engine includes a error checker to prevent the all potential problems. You have an explanation of the problem and how to solve it. In addition to the possibility in certain cases of correcting the problem automatically.

This doc is just a list of potential error with explanations. In progress...

Unreal Engine support only one root bone in a Skeletal Mesh. If the armature is not named "Armature" Unreal Engine will create a new root bone. This is wrong because the root bone will have a scale of 100 and this produce physics errors.

For fix the multiple root bones: Add a new bone to the Armature and name it "Root" then attach the other root bones to the new bone.

Note: With my addon all skeletal mesh are exported with the name "Armature" for avoid physics errors. You can change this in Addon preferences.

More info here: Skeleton & Root bone

Bendy bones are not supported by Unreal Engine, so that better to disable it if you want the same animation in Unreal Engine and blender.

If you use Parent Bone to parent your mesh to your armature the export will be wrong.

More info:

• https://docs.blender.org/manual/en/latest/sculpt_paint/weight_paint/usage.html
• https://www.youtube.com/watch?v=dKZrzG5r13g