I am not quite sure about your issue. If you are using Captcha in Web Farm or Web Garden environment, the captcha image cannot be accessed since the application runs on more than one web server at the same time. To avoid this behavior, you should store the CaptchaImage in the
Session, and configure your server environment to use out of process
Session State. Check the following help documentation for more.
Using RadCaptcha in a WebFarm/WebGarden Environment