Since Q3 when a RadWindow is declared in RadWindowManager it preserves its ViewState which was not so in previous versions. This could lead to backwards incompatibility when the VisibleOnPageLoad property is used in this configuration with the idea to show the RadWindow only once. Possible solutions for this case are the following ones:
* Set EnableViewState = "false" for the RadWindowManager
* Reset the VisibleOnPageLoad property to false with code when suitable, depending on the particular scenario
* Show the RadWindow through registering a script from the server instead.
the Telerik team