We're currently upgrading our implementation of the Classic RadComboBox to the Ajax version, however after trying to assign the combobox SelectedIndex and Text properties with the below code we get the above error message.
cmbCats.SelectedIndex = cmbCats.FindItemIndexByValue(
cmbCats.Text = cmbCats.SelectedItem.Text;
We're using a List as the datasource object and if the above code is omitted the error is avoided, but obviously the correct item isn't selected.
I hope this is enough information to help you diagnose the problem.