reCAPTCHA is blank or returns "Input error: Invalid referer"
If you set up reCAPTCHA in the admin but you get an "Input error: Invalid referer" error or the captcha image does not display, this is most likely due to incorrect keys. It could also because you are trying to use the reCAPTCHA on secure areas of your site (such as the admin login or account creation) but are using a shared SSL (rather than a dedicated SSL).
When you sign up for a reCAPTCHA account, you must enter your domain, and since a shared SSL (e.g. https://www21.corecommerce.com) does not use your domain, it breaks the connection with reCAPTCHA. For further information on obtaining a Dedicated SSL, please see the Related Articles below.