Steps to reproduce:
Fail Text displays:
"This field is required. Please enter the letters shown in the image."
ReCAPCHA V2 uses a check box and image system so this text does not apply and often confuses our users.
Just a guess, but on enabling V2 of Recaptcha, did you insert the correct site and secret keys for V2? It won't work with the public/private key of V1 - you need new, specific site/secret keys for V2 (obtained from your Google account).