The issue may be caused because the scripts of RadCombobox have not been loaded properly.
Please make sure that you have upgrade to the latest cumulative update
of the SharePoint 2010 server - it came out at the end of October 2011.
The cumulative pack contained a fix for an issue where SharePoint 2010 was failing to load scripts on an Ajax request.
Perhaps, this is the case in your scenario as well.
Hope this will be helpful.
the Telerik team
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