I\'m using the new Google reCaptcha API for a contact form inside a page on my wordpress instance, but the API returns a 500 Internal Server Error message
you got this error because you didn't put in your domain into Key Settings in recaptcha admin site or basically wrong domain.
I've solved the problem, but the funny thing is that I don't know how, I've just re-copied the code from Google Documentation, maybe there was a mistyping.
Anyway, problem solved.