Ronald,
It seems to work here. But there might be some extra reasons.
Could you please provide some additional code on the way you bind the treeview and get access to ContentContainer.
I've managed to create a page with the following code:
OnSelectedNodeChanged=\"ftv_SelectedNodeChanged\">
And the handler:
protected void ftv_SelectedNodeChanged(object sender, SelectedNodeChangedEventArgs e)
{
Control control = ftv.Nodes[1].ContentContainer.FindControl(\"myLabel\");
}