I am sorry to hear you are running into this problem. You didn't specify what type of Combobox you are testing. Is it a Silverlight or an Html Combobox? Is it a generic control or a RadControl?
We'd like to see the error first-hand to diagnose it. Please see here
for what information to provide. If you can grant us access to your app but feel that information is sensitive, you can submit a support ticket which is confidential, unlike this forum.
If you can replicate the issue against a public site, like this Telerik demo site
for example, let's continue the discussion here. Please provide reproduction steps in that case.
All the best,
the Telerik team