To reproduce:

public RadForm1()
{
    InitializeComponent(); 

    for (int i = 0; i < 10; i++)
    {
        this.radTreeView1.Nodes.Add("Node1." + i); 
    }
    this.radTreeView1.AllowDragDrop = true; 
    
    this.radTreeView1.TreeViewElement.DragDropService.PreviewDragDrop+=DragDropService_PreviewDragDrop;
}

private void DragDropService_PreviewDragDrop(object sender, RadDropEventArgs e)
{
    e.Handled = true;
}