Try -
Call CatSubProduktAreakum(Stattyp, Daty + UBound(SubCategories) + 2)
As for the reason, this from MSDN via this question - What does the Call keyword do in VB6?
You are not required to use the Call keyword when calling a procedure. However, if you use the Call keyword to call a procedure that requires arguments, argumentlist must be enclosed in parentheses. If you omit the Call keyword, you also must omit the parentheses around argumentlist. If you use either Call syntax to call any intrinsic or user-defined function, the function's return value is discarded.