Connection inside of a spline. SC can't follow.

[Trainzland]

SplineControllers don't follow connections, event handlers do by setting the controller to a new spline. So I need at least the code of the event handler you're using. Connection settings would be of help as well.

The Connection settings prevent to set the FollowUp (or Heading however) into the red arrow direction. See image below.
Your new Event Handler (UseFollowUp) you provided works in this case. But crash again in other situations unfortunately.

Anyway..
This situations while curvy produce bugs (mostly while editing Connections) and cause unity to fail are complex. Explaining the complex stuff seems  waste of time currently,
because it there is none who's willing or interesting to understand this complex fails. Laying a simple spline from A to B works. Working with Connections produce too much fails to get in basic workflow. I would recommended to spend more time into the editing Connection features to keep them running better.