The RadCaptcha renders grey image when the image with the validation code cannot be found in cache/session. This could be a result of:
- Using RadCaptcha in a WebFarm/WebGarden Environment
- The cache in you website being cleared for some reason. You could check if this is the case by storing some values in Cache and retrieving them.
- The life cycle of the page, in which the captcha is placed, is executed more than once due to the reason described in this blog post
the Telerik team
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