I can see it in your video, that you are still in animation mode at 35 seconds into the video, when you perform the export.
Just checked your x file in assimp viewer and it is rotating the whole model not just the arm.
EDIT:
Can you use 2 bones instead. Start from 0,0,0 and drag the first one up to the bottom of the split of the base of the barrier.
Extend the next to the pivot point. You may need one more.
The reason why you cant see the base is because it has no bone assignment and the shader will only render objects with bones. Which shader are you using? If on PBR then use the apbr_anim.fx shader.
Select the verticies of the base and assign to the first bone. Select the vertices of the arm and assign them to the rotating pivot or bone.
Also from the Fragmotion project file you sent me, there is a vertice on the long end of the arm that is not assigned to the rotating bone.
Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz, 2400 Mhz, 4 Core(s), 4 Logical Processor(s), 8gb RAM, Nvidia gtx660, Windows 7 Pro 64bit, Screen resolution 1680 x 1050.
