There was a similar error in the previous versions of RadComboBox. This problem is fixed in 2008.2.826 and we suggest upgrading to that version.
Alternatively you can try the following:
Set the EnableAjax property of RadAjaxManager to false.
Try using MS AJAX instead of RadAjax
Call the attachDropDown
method of RadComboBox before the ajax request. Read our help article
for more details.
You can also prepare and send a sample running project illustrating the problem.
the Telerik team
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