Basically, when a RadWindow is closed, it remains hidden on the page (so it can be shown quickly on subsequent calls). In some cases, developer needs this object to be destroyed and that is where the DestroyOnClose property is used. When it is set to true, once closed, RadWindow's object is destroyed and you cannot use that object again until the whole page is reloaded.
I am still not sure though, why setting this property to true causes the problem in your setup - as I said, the logic is pretty common and so far I am unable to reproduce it locally. This leads me to believe that the reason is somewhere else, but without having a better view over your exact setup and logic, I cannot tell what is the cause.
I will be grateful (if you have the time of course) if you send me a sample project where the problem can be reproduced so I can examine it. If you decide to do that, this blog post
would be of help.
the Telerik team