Our joomla site is using K2 for the user registration page. In the K2 settings, I had "Enable reCaptcha on registration form" set to Yes, "Use reCAPTCHA v2" set to Yes, and the public/private keys set up. With these settings, the recaptcha box would not show up on my registration form and registering would give the following error: "Empty solution not allowed"

I read in other forum post that you cannot have recaptcha enabled in both K2 and the default Joomla users plugin. So I tried turning off recaptcha in the K2 plugin by setting "Enable reCaptcha on registration form" to No. With these settings, I get the following error when trying to register: "Could not verify that you are not a robot".

What do I need to do to get this working? Turning off recaptcha in the default Joomla users plugin is not an option because my event registration component relies on it.

By default the setup you need to follow is:

Enter you keys for K2 and enable captchas for the registration form.
Select v2 captchas (you need the latest version for this).
Go to the users component and disable the captchas since K2's captchas will be shown.

You mentioned the event registration component, is this is a separate registration component or it based on com_users?