The problem might come from the fact that you perform an ajax call before entering the code an submitting the page. When an AJAX request is fired, the server-side page goes through its full page lifecycle. This means that a new RadCaptcha code will be generated, but because it's an AJAX update it would not be shown on the image (the old code is still shown because the RadCaptcha is not affected by the partial page update). So, the user sees the old code and enters it correctly but still gets an Error Message, because this old code is not valid anymore. The easiest way to overcome this is to place the RadCaptcha control inside an UpdatePanel (or AJAX Panel), so that the code is updated when AJAX update occurs.
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