Trying to open a RadComboBox with

var csBox = Window.Find.ByName<RadComboBox>(ComboBox);
csBox.EnsureClickable();

csBox.SelectItem(1);

Observing the automation shows we are waiting no time for dropdown to open and I am getting exceptions claiming there are no items in the list.

The cause is related to the fact that the public void WaitDropDownAnimation(int millis) method of Telerik.WebAii.Controls.Xaml.Wpf.RadComboBox is not working as expected. 

Workaround: You can use Window.Find.ByType<RadComboBoxItem>().Wait.ForNoMotion(milliseconds); before selecting the item from the dropdown.