This problem is caused by the RadRibbonDropDownButton's autoclose behavior - it listens for a SelectionChanged event and when such is raised, it closes its dropdown. In your case, when RadComboBox changes its selection, the DropDownButton closes its dropdown, but RadComboBox tries to scroll itself, so the newly selected item is visible and fails with the exception you are seeing.
To workaround the problem we would recommend setting the KeepOpen property on RadRibbonDropDownButton to true:
<ribbon:RadRibbonDropDownButton KeepOpen="True" ...
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