Since I am not familiar with the project at your end and the implementation that you are using, I cannot provide you with an exact answer what is causing this exception. However, from the demonstrated errors it seems that the problem is related with the RadScriptManager and the proper loading of the Resources. Could you try to replace the RadScriptManager with the asp:ScriptManager, in order to make sure that the issue is related with it?
In addition, here are some forum thread, which you might find helpful in resolving the problem:
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