This is an odd issue and I would advise first going through this article: http://www.telerik.com/help/aspnet-ajax/window-troubleshooting-missing-borders-titlebar-and-commands.html
. Most likely, something is blocking webresource requests in the problematic browsers, so either finding what that is, or using the CDN should help.
You can also look into using the lightweight rendermode because it uses less image spites for backgrounds, so this issue is less likely to manifest: http://www.telerik.com/help/aspnet-ajax/window-render-modes.html
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