Hi. I'm having the same problem: I installed the HikaShop ReCaptcha v2 plugin, and everything seems to work fine until the user presses the register button. Then the following error appears: "We couldn't contact the reCAPTCHA server to verify your answer. Please try again."
The user is created, but no email is sent, so there's no way to finalize registration.
I uninstalled the Joomla recaptcha module and checked in all the places I could find to make sure it was disabled everywhere (there's plenty of info on that if you Google "turn off the joomla captcha plugin"). Anyway, I'm dead in the water right now. No way to allow user registration without completely disabling captcha, meaning that I have to spend my life deleting fake users created by spambots.
Any help would be greatly appreciated.