Character Skinned mesh Anim breaking

When i get this error task.vertexbuffer == null either one of my enemy character animation breaks or my player charater’s anim breaks.
Its like the bones are animating but the mesh is not in sync with it, can anyone help me with this?
I am not able reproduce this always, it happens randomly.

PS: i am using legacy animation

The same problem.
unity produce task.vertexBuffer == null randomly.

And some skinned animation is broken. (we have multiple skinned animation objects in a scene)

I am using humanoid not legacy. Unity version 5.3.8f2.

Any idea?