My Unreal Engine cashes when i import animation

crash report:
Assertion failed: (Index >= 0) & (Index < ArrayNum) [File:D:\Build++UE4\Sync\Engine\Source\Runtime\Core\Public\Containers/Array.h] [Line: 614] Array index out of bounds: 0 from an array of size 0

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when i import just a static mesh it works but when i import it as skeletal mesh with animations it somehow crashes.

i tryd to export my fbx as simple as possible and my blender file is also verry simple just a mech and amature bound.

it must be unreal engine that crashes somehow because of something i dont know what it is but it must be a simple issue.