I figured out the problem, just by chance!
I did not have the variable used on the datcard.
As soon as I linked the variable to an eidt-box on the datacard it worked!
Doesn't make sense to me because the variable is linked in the tab properties "Controlled by variable".
I really don't want to see the variable on the datacard since it is just used in the background to control the tabs.
I tried turning off the "Show in Explorer" but the Edit-box still shows in the datacard in Windows explorer!
In the card editor, you can make a box hidden or greyed out by using the "Control Logic" feature.