Generally speaking this error can occur if you are changing the controls collection on the server. At this point we cannot tell what the exact reason for the problem is, but we believe that the problem is not directly related to the RadWindow control.
To be able to help, we will need a small sample project where the problem can be reproduced - we will check it and get back to you right away. Just open a new support ticket and send the project to us.
the Telerik team