Generally placing the RadWindow in an update panel is not recommended, because the rendered HTML is moved
from its original location in the markup. This will most often result in
the expected functionality simply not working.
Actually your problem is that you add the OnClientClick handler in the server click handler itself. You need to add it in the markup or in Page_Init. With your current logic the client click takes place, not logic is attached to it, the button posts back, it server click handler is hit, client click logic is added, so the next time you click it there is client click handler.