WPF datagrid vertical scrollbat does not work after set another control

Question

I have a WPF MVVM program with a datagrid dat is populated with
records. Works fine. I have a label which is bound to a property in the viewmodel (INotifyPropertyChanged).
Works fine, but when i change the text of the label through the property, the vertical scrollbar of the
datagrid doesnt work anymore. I can still select rows in the datagrid and scroll through the rows with mouse
but the vertical scrollbar is frozen. Very strange. Has anybody any idea what can be the problem.