This code seems outdated:

http://www.unifycommunity.com/wiki/index.php?title=Spline_Controller

Is there anybody skilled enough to fix it or is there a fix somewhere available?

You are trying to create a MonoBehaviour using the 'new' keyword.  This is not allowed.  MonoBehaviours can only be added using AddComponent().  Alternatively, your script can inherit from ScriptableObject or no base class at all
UnityEngine.MonoBehaviour:.ctor()
UnityEngine.MonoBehaviour:.ctor()
SplineInterpolator:.ctor() (at Assets\AI\SplineController\SplineInterpolator.js:1)
SplineController:OnDrawGizmos() (at Assets\AI\SplineController\SplineController.js:24)
UnityEditor.Handles:Internal_DrawBuiltinGizmos()
UnityEditor.Handles:Internal_DrawBuiltinGizmos()
UnityEditor.Handles:DrawBuiltinGizmos()
UnityEditor.SceneView:OnGUI()
System.Reflection.MonoMethod:InternalInvoke(Object, Object[])
System.Reflection.MonoMethod:InternalInvoke(Object, Object[])
System.Reflection.MonoMethod:Invoke(Object, BindingFlags, Binder, Object[], CultureInfo)
System.Reflection.MethodBase:Invoke(Object, Object[])
UnityEditor.HostView:Invoke(String)
UnityEditor.DockArea:OnGUI()

[..\..\Runtime\Export\Generated\BaseClass.cpp line 1029]