I apologize but browsing your forum I did not find a solution to my problem, so I try to write to you.
In the meantime, thank you for publishing this component for free.
I have installed the CMC component on a Joomla 3.9.23. I have correctly configured the connection to Mailchimp so that the registrations of new users are also registered on the selected audience. Great!
The problem exists when a user tries to unsubscribe from the newsletter from the Joomla control panel. The "Newsletter" checkbox is not set (and instead it should be?!?) and if instead we set it and then save the error arrives:
COM_CMC_SUBSCRIBE_FAILED 400: <EMAIL ADDRESS> is already a list member. Use PUT to insert or update list members.
Question: is unsubscribing to the newsletter from the Joomla panel required?
My CMC module configuration is as follows:
On the main settings -> Use double opt-in YES
On Plugin User - CMC Registration -> List <customer-list-name> only.