how do I position a splineController on a spline in runtime to an exact position?

[_Aka_]

My guess is that you are giving a relative distance while your controller expects an absolute one.
Position sets/gets either the absolute or relative position (TF) depending on your controller's MoveMode.
So either set that mode to Relative, or use mySplineController.RelativePosition, which always takes a relative position, regardless of the controller's MoveMode.