Hello Peter Meinl,
From the code snippet you provided I couldn't see the type of your ItemsSource collection. If you're using a List collection, the changes you make to the RadTreeView ItemsSource won't affect the original business object, which could cause the problem. If this is the case then you should use ObservableCollection instead of List.
I have prepared a simple example illustrating how you can populate your RadTreeView with data from code behind. I am using ObservableCollection for the ItemsSource of the RadTreeView. I have added a Delete button that deletes the selected item of the RadTreeView. Please take a look at it and let me know if it helps.
Please let us know if you need more info.
the Telerik team
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