I was pleased to find your plugin as a client I'm working with uses CC. I installed and used the correct CC credentials (I can login with same on CC site).
This is the error I'm receiving:
Invalid username / password combo. Please try another combination. Please note: spaces in your username are not allowed. You can change your username in the My Account link when you are logged into your account, and this may remedy the problem.
There are no spaces in the username but there is an @ symbol. Unfortunately, as this is a client site, I'm unable to change their username at this time.
Do you think this could be something that would affect your plugin's ability to connect to CC?
I'm an experienced WP user, so any troubleshooting of manual plugin edits to test any ideas you have are doable.
I am looking forward to any advice you may have so I can move forward with your plugin. And as always, I appreciate you making this available on the repo:)