Overwrite formula in cell with Button-Parameter

[schlemiel29]

I'm facing the problem, that I set an initial value (actual month from NOW() formula) with a simple formula. As long as there is a formula, this value will not be overwritten by an action of a button and its parameters in destination object. But when I overwrite the formula with a value from a validaded excel list or simply type a valid month name in, then updating of this cell works by the properties set in the dialog of the button. The cell itself is not write protected. This behaviour only happens in TM1WEB, not in Excel itself.

So what is the reason, that a cell with formula can not be replaced by a value defined in "Extended options" (sorry, my orginal text is in german)?
Is there a work around?