The problem you described appears to be similar to an issue with RadCaptcha when its validation code is stored in the Cache of the application that was fixed in the Q2 2013 release of the Telerik controls (version 2013.2.611).
The issue was that when you open the page with the captcha on another browser tab or window, a new validation code is generated and the code that is displayed in the original tab/window you opened is no longer valid.
The best way to resolve this issue is to upgrade the Telerik controls to the latest version available. It is also possible to avoid the problem at hand by switching the ImageStorageLocation
property of the captcha to Session
. Note that in this case you also need to change the way Telerik.Web.UI.WebResource.axd
handler is registered as described in the following help article: http://www.telerik.com/help/aspnet-ajax/captcha_getting_started.html
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