Unfortunately the error you are facing is too generic and it is not really possible for us to know the exact reason for it. It is possible that the type your returning is exposing a non supported CLR for example, or it is not matching all the requirmenets OpenAccess might have.
In order to determine the error you can subscribe to the Completed even of your LoadOperation. In the call back method you will be able to observe the actual error your service has thrown and thus have a better knowledge on how to fix it.
the Telerik team
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