Hi Christian,
I've not tried this particular event before but I do know there are events and methods that appear in the COM model which don't work (whether they are buggy or just were never completed, I'm not sure).
I recommend using the Validate event (BeforeAction = false) for checking whether a value has been changed by the user. One thing you do need to know about the Validate event is that it doesn't fire in some cases when the user right-clicks another control while the focus is currently on the item you want to validate. Therefore, it's always best to have a global check, if possible, to double-check that the data is correct (eg recheck your UDF value on each row when the user clicks on the Add/Update button).
Kind Regards,
Owen