Hi Sasa,

Such behavior is expected - you will get the same result if you don't use RadWindow, but a standard popup or load the content page in an IFRAME. In all cases, when you open a page while your session has been expired, you will be redirected to the login page.
To avoid the problem, I would suggest to add a "frame buster" script to the login page that will ensure that this page will always be opened in the browser window.
For example:

The logic behind it is simple - if this page is not displayed as a topmost page (e.g directly in the browser) but in some container (like IFRAME / RadWindow /etc.) - load it as topmost page. This way when your session is expired and you open a page in a RadWindow, you'll be redirected to the login page inside the RadWindow. Because of the script above, however, the login page will be loaded as topmost - directly in the browser's window.

Sincerely yours,
Georgi Tunev
the Telerik team