I've pasted a code snippet from within RadComboBox.PerformDataBinding. Is there any reason that the ID of the dropdown is not put in the exception? This seems like a very easy code change that would help many people tremendously in debugging binding issues when they have multiple dropdowns.
I believe an OOTB ASP .NET dropdown box will also show the ID on a binding exception.
RadComboBoxItem selectedItem = FindItemByValue(cachedSelectedValue);
"Selection out of range"