We have a check that makes sure that you are only updating data that is already loaded. Because your code is an Add(), I am not sure what really goes on, maybe you have a reference to an already persistent object inside your new object.
It would help me to did further into the problem if you can send me the complete stack trace with all inner exceptions. You can create that by just calling exception.ToString() in your application or in the details window in the debugger.
the Telerik team
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