Arm joints won't work properly

So i tried to follow Polyfjords last tutorial (https://www.youtube.com/watch?v=MtlLUdUvw-g&t=22s) but unfortunately i didn’t get it work.

The rig is basically download from the web and has a keyframed animation on the bones.

My problem are the joints, which look sometimes/often really bad like this:

image

or this:

image

here is my blend file:

https://www.dropbox.com/s/5ywwzc0u3d0viwo/pf_tut_rig_problem.blend?dl=0

the blend file consists only of one arm, but by this you can see the problem.

e.g. frame 484:
image

Can anybody help me how to solve this?