Adding a Physics Asset to a Skeletal Mesh causes it to rotate by 90°

We have created a 3D model in Blender and imported it in UE4 as component for our player character actor.
The model is aligned the X axis in Blender, it also aligns perfectly along the X axis in UE4.

Then we manually created a physics assets for the model within UE4, in order to enable collisions and be able to apply forces to the model. That physics asset is quite simple, it only contains 3 physics bodies for the 3 bones in the model. However, as soon as we set this physics asset for the skeletal mesh in UE4 and launch the game, the actor/mesh is rotated by exactly 90°. It looks as if the physics asset is set for the mesh, the actor/mesh is aligned the Y axis.

As soon as we remove the physics asset from the mesh and launch the game, everything works fine again, the mesh is aligned correctly along the X axis.

I don’t really know where to look for the cause of this issue.
Do you have any ideas what could be wrong here?