Ping pong game, ball moving like classic

This may not actually answer your question, but you have an error on your third condition down where you are comparing Ball_objekt.Top <= playground.Top. You are then reversing the ball's horizontal speed (speed_left) rather than its vertical speed (speed_top).

 if (Ball_objekt.Top <= playground.Top)
 {
    speed_top = -speed_top; // <-- here
  }