"Input error: k: Format of site key was invalid" when reCAPTCHA is turned on

edit

Have the exact same problem and took the liberty of taking a look at the HTML code of the page and the settings.py of the recaptcha module. I think I found the source of the error:

_RECAPTCHA_HTML = u'''
%(options)s
<script type="text/javascript"
   src="%(proto)s://www.google.com/recaptcha/api/challenge?k=%(public_key)s">
</script>
<noscript>
   <iframe src="%(proto)s://www.google.com/recaptcha/api/noscript?k=%(public_key)s"
       height="300" width="500" frameborder="0"></iframe><br>
   <textarea name="recaptcha_challenge_field" rows="3" cols="40">
   </textarea>
<input type="hidden" name="recaptcha_response_field" value="manual_challenge">
</noscript>
'''

As you can see, there are three variables options, proto and public_key supposed to be rendered into the string, but nothing is defined!

A quick and dirty solution is to replace %(public_key)s with your actual public key...

One quick thing to check is if you switched from an AJAX display for your captcha to a PHP based display and left in:

showRecaptcha(element)

Then comment out that function / function call, took me a while to figure out because it would only error out every 2/5 times depending on how fast the JavaScript loaded.