With respect to this forum thread there is no "reload" method for content template containers (in the framework) and the main reason for that is that there is no way to know what exactly the user /developer wants - should the content be reset, or should it preserve some values.
One suggestion is that you can refresh the ContentTemplate manually and you need to put the update panel directly in the ContentTemplate as shown below.
But for some reason this approach is not working for me. I am getting the error below. But I could rebind RadGrid and open RadWindow from the sever side code.
Cannot unregister UpdatePanel with ID 'Updatepanel1'since it was not registered with the ScriptManager. This might occur if the UpdatePanel was removed from the control tree and later added again, which is not supported.
Unfortunately i couldn't replicate the issue that you are facing at my end. After having some research I found that this forum thread discuss the same issue. Please take a look into that and let me know whether it helps.