Physics asset issue - some Body parts Shaking, Head bone detached

Like title says, Here’s a picture of default body :

If Picture is not enough you can check the asset file :…cs.uasset?dl=0

And here’s a picture of physics simulation :

as you can see head bone seems not connected to the Body like the rest of the bones, also some of the bones are jumping like crazy, I don’t know which ones are responsible of this, And I don’t know how can I attach the Head bone like the rest are, any idea what’s wrong with it ? Thanks.