ok it works with your blend file, I re exported the fbx
now I gotta figure out why my model is not working 
I am gonna restart from scratch
thx Limanima
You don´t have a skeletal do you?
I got many things in my blend, I also got linked blend files with skeletals
but I only export the select nodes
If you want send me the blend file and I’ll check it.
Edit:
Not sure if it can start with “_00”
It does, you can even import only a single collision mesh with _00
yeah I tried 00->01 , same problem anyway
I simplified my blend
juste one mesh, its 4 collisions, removed everything else
it still regenerates the collision instead of using mine 
It’s kind of weird, now that I look at the documentation, it states UBX would be a rectangular prism, which that definitely is, but I do remember testing it some time ago and basically anything but an uniformly scaled box would get rejected. Even the documentation with the lollipop example uses UCX for something that is a rectangular prism.
EDIT: Ah, now I get it! You cannot move the vertices around in any way even if it remains a rectangular prism, but you can scale it non-uniformly, as long as it is a rectangular prism. So therefore even the lollipop box in the example could be UBX. Great, I’ve been using UCX so far for a lot of collision meshes that could be UBX, though I don’t know how much of a performance difference that makes. I’d assume UBX is somewhat lighter.
Therefore, you’re fine using UBX as long as you’re only scaling the object in object mode. Don’t enter edit mode at all. EDIT: nvm, just keep it rectangular and it’s fine
And I suspect if you want to rotate the box, you can’t apply rotation or else it will not be seen as a box anymore even if it’s still a box. But I’m not sure of this…
… by definition, a convex hull is a parallelepiped - shrinking only one face of a box Is a parallelepiped, definitely not a box anymore.
Same reason the tower I shared has to use UCX essentially…
You can definitely rotate the whole box and manipulate scaling however you see fit…
You can rotate the box, actually, you can also move the vertices in edit mode, I guess it’s just extremely touchy about it? Not actually even sure but UBX seems to be working fine right now for me.
Just really weird that even the lollipop example uses UCX instead of UBX, I do remember having issues with UBX but testing it now it seems to work for any rectangular shape.
Did you applied rotation in blender?
Yes you did, so rotate the boxes is not a problem.
Yep. His rectangles didn’t conform to the shape, because the sides were slanted. You actually highlighted that in the small image. I somehow missed that.
I just vaguely remember testing that UBX should’ve been uniformly scaled Box, even the example uses UCX where UBX should suffice. Turns out you don’t even need to keep the origin in the center for the box to be accepted. Not sure if it’s the engine, blender, or something that has changed since. Well, good to know at least.
it does not look like my model…besides I created blender cubes, resized them, then applied transforms
wow, it does not get touchier than this
I’ll recreate the boxes
If you want to know why it’s so touchy, open up the mesh editor, edit the collisions, select the hull, and try to manipulate the numbers of a UBX.
You can also create the ubx in engine.
Either way, it should become clear how it works 