Thank you for the fast answer jwatte.
-edited-
Morning, I shouldnt post late at evening - even I can’t make much sense of my text 
Here is another try:
Since I put all the calculations into functions set by timers (0,01s) I thought I would sidestep the problem with the fps.
The biggest problem while playing over the internet is the calculation of friction and my ‘drift force’ since those are calculated on the base of the current velocity.
I understand, that those calculations depend on a stable server to run properly. But what I do not understand is, why those hickups occur even while Server runs on 120 fps
and doesnt show any signs of slowing down.
When I let all my calculations be run on the server side, shouldnt i just get a laggy experience on the client side instead of physics going insane?
I am planning to rewrite all my movement and replication system to do the now serverside calculation on the client side, put the replication aside and let the clients just
tell the server their position and let server handle passing that information down to other clients.
But since iam an absolute beginner in everything related to the topic of gamedesigning I have no idea if the path iam trying to take doesnt end in a dead end.
Any suggestions on how to work on this issue would be most appreciated
greetings, fels
-edit2-
Further searching for information on that topic brought me back to substep the physics thus altering the “wrong” delta time for lower fps rates. I did use it already, but must see now, that I misunderstood the concept behind it 