Google has changed javascript URL. This affects both Joomla reCaptcha and K2. To make it to work I did this:
Joomla plugin:
Edit plugins/captcha/recaptcha/recaptcha.php and change URL
Line 24: const RECAPTCHA_API_SERVER = "http://api.recaptcha.net/recaptcha/api";
Save file.
K2:
Edit components/com_k2/views/item/view.html.php and change URL
Line 168: change URL to "http://api.recaptcha.net/recaptcha/api/js/recaptcha_ajax.js"
Save file.
Done! Working for me now!