Weird error when registering

If the recaptcha plugin is disabled, the registration works fine. With it active, if a user doesn’t fill in the captcha, the plugin works as expected (user gets ERROR: Please fill in the reCAPTCHA form.)

If a user fills in the captcha, email and username, the error element shows up but without the error text and the registration doesn’t happen. Can anybody take a look and tell me what the problem is?

http://searchplast.com/wp-login.php?action=register

Thank you

https://wordpress.org/plugins/wp-recaptcha/