skeletal objects cast inconsistent shadows


I have a Level set up where a volume spawns rigged skeletal hand meshes which fall on a rotating stone. However, a irregular amount of these hands cast weird shadows (?) on themselves or generally appear darker when entering or leaving the frame of the screen. I have tried to change all kind of render, lightmass and post-process settings and i have still no idea why this problem occurs. I previously run this level on a mac mini without GPU on lower graphic settings without this problem occuring. I have the feeling i am missing something very obvious, but i can’t figure it out. any ideas?