Hello Marco,
Are you using the ContentTemplate of the RadWindow? If so - a postback that originates from inside will close the RadWIndow, as the RadWindow is created, opened and closed via JavaScript and cannot survive postbacks. What you would need to do in this case is to use AJAX to avoid disposing the entire page. How to integrate an UpdatePanel with the ContentTemplate of the RadWindow is explained in the following help article:
http://www.telerik.com/help/aspnet-ajax/radwindow-ajaxifying.html.
As for the invalid arguments exception - it can be due to different factors - incorrect attempts to use AJAX, incorrect programmatic creation of the grid (this is the most probable - it is possible that is is created too late and not in the Page Init event and does not have the same ID set). In case doing things correctly does not help I advise that you open a support ticket and send us a small sample that can reproduce the problem so we can debug it and see what is going on.
Kind regards,
Marin
the Telerik team
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