I noticed that you have opened several threads about the same issue. Please, refer to the support ticket post for additional information. To avoid duplicate posts, I suggest you continue the communication there.
However, attached to this message is a modified project which is working properly. Give it a try and let me know if other questions or problems arise.
In order to overcome this error you should open the window on client as in the modified project .
Additionally, I suggest you avoid ajaxifying the RadWindow control.
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