When Export from UE4 to Blender as fbx. IK Bones are gone and the skeleton from my character changed normally I don't ask for solutions and try to solve problems myself, but I'm stuck for several days. I created a VRoid character and first exported it to UE4 with a plugin, then I followed a tutorial https://www.youtube.com/watch?v=fsP68yQHIbs. So now comes my problem with Blender. When I export my character as fbx to Blender. Blender automatically deletes 3 things and sends me a traceback error. 
My IK bones are gone, and my skeletal mesh is changed. If I export another character like the ALS V4 Character to Blender, everything is fine.
The IK bones were still present. I just had to rename them. To be honest, I still don't know what Blender deleted and why the traceback error occurred, but at least I still have the IK bones and the other bones. The best part is that the bones are even in the right position. :D