"Failed to Re-import Assault_Rifle_A (SkeletalMesh)"

I’m trying to use the assault rifle from the Military Weapons pack in the First Person Shooter example game (not the template – the nice-looking indoors atrium level/game)
The assault rifle comes in oriented wrong, and needs to be rotated 90 degrees negative around up.
So, I export the rifle to FBX in the project/content folder, and import this FBX into 3ds Max 2014.
In 3ds Max, I rotate the root bone 90 degrees and offset the gun a bit to line up with where the handle in the FPS game is for the other weapons.
Then I re-export the mesh as skeletal mesh in FBX, overwriting the FBX that I initially exported from UE Editor.
Then I attempt to re-import the mesh, and navigate to the exported Assault_Rifle_A.fbx file. However, this import fails.
Trying to re-import again, I don’t need to navigate to the FBX (it has remembered the path) but still fails.
The only error message I get is the message “Failed to Re-import Assault_Rifle_A (SkeletalMesh)” in a floating bubble in the lower-right corner – no log messages in the output log window.
I’ve tried this after exiting 3ds Max, just to make sure Max isn’t holding on to a file lock.

How can I debug this? What am I doing wrong? How can I figure out what’s failing?

you need to check if you have errors on export
don’t forget when you export a skel mesh from UE4 it adds an extra bone to the root of the rig (standard pain in the a*** fbx exporter) so you may have to delete this before you import the skel mesh back into ue4