Hi Ozcan Oksuz,
This seems to be a multithreading problem, you are using the same context object in two threads at the same time. I am not sure about you architecture, but if this is a web or application server process you should use a new scope per request or at least per thread. I prefer to have a context or data access layer instance in the master page and dispose it in the masterpage dispose:
the Telerik team
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