Movement Stutter

Hi Guys,

i have uploaded a (very short) video showing my problem, which is best watched with 720p60fps:

You should recognize, that the movement of the player character is “stuttering” on a frequent rate.
I have two totally different player pawns and the issue occurs on both, so it should have nothing to do with the way of input.
if you keep an eye on the ball, it is hard to tell, but i want to believe it is stuttering as well.
the entire issue is amplified with lag over the internet. the client has insane stuttering all over the place.

what could cause this?

EDIT:

the problem only seem to occur in standalone or full build, not in selected viewport!

r.VSync 1 solves it, but causes a bit of input lag