Hi,
it seems that in the presets for creating a Custom Gantt from a Global Table, the reference to the associated tables is declared with the Object class instead of the Table which throws the exception:
exception: FlexScript exception: Invalid down cast. Object is not an instance of the target type. at MODEL:/Tools/Statistics/Custom Gantt>variables/numseries
At least in a simple test I'm doing, changing the class solves the exception.