Number variable behaving erratically since last update?

Hello,

I'm running into problems with navigation in lightboxes.

Our setup is as follows:

• A main scene with mostly question slides and a second scene with Information slides, always presented in lightboxes 
• Information slides can consist of multiple layers: if so, these layers are shown or hidden after the user presses a custom shape.
• When the shape is clicked a number variable is increased or decreased. (trigger on the master slide)
  Depending on the actual value of the varaible, a layer becomes visible or hidden. (trigger on the slide)
  This value of the number variable is initialized on the slide master

Demonstration of the problem:

In the attached storyline file, the navigation shape on the lightboxed slide does not trigger the layer. Usually this occurs sometime after going through a couple of slides and lightboxes.

This can be demonstrated by,

1. going to a question 
2. clicking the green text (lightbox opens)
3. navigate to the last layer
4. close the lightbox
5. repeat for 2 more question slides
6. sooner or later the lightbox that opens will stop showing the layers 

Hypothesis:

From my tests, it seems as if the number variable is counting erratically; when clicking the navigation buton and closing and reopening the lightbox I seem to get random numbers representing the variable.

Anybody else having problems with number variables? Is this something I can fix myself? Thanks in advance,

Bart