Hello, we just installed this component and ran into an error right away. We followed the configuration steps. The on-screen instructions were a little different than the documentation on the website. When we first visited the configuration screen I pasted in my existing mailchimp api, then hit the OAuth button. That then presented me with a login screen, which I assumed was for my mailchimp credentials. After entering those a new api key was displayed and the instructions said to enter that in the mailchimp API field (the one I had already entered). So I did that and everything seemed to work. I was able to see my existing lists and users from mailchimp.
So now the error: I opened up a user in the joomla user list and went to the Newsletter tab and checked the box. When I hit the Save button I received this error:
COM_CMC_SUBSCRIBE_FAILED 400: The resource submitted could not be validated. For field-specific details, see the 'errors' array.