Is the Ajax modal popup extender making an ajax call before showing the RadCaptcha control? 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.
To overcome this, you should place the RadCaptcha control into an UpdatePanel with UpdateMode = Always. This way you can be sure the captcha has the latest code displayed.
All the best,
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items