To ensure synchronization between Mailchimp and Shopware, the plugin registers a webhook with Mailchimp.

To check the webhook, Mailchimp calls up the corresponding URL/domain of the SalesChannel. If Mailchimp cannot access the URL, an error occurs. This is often because the plugin is being tested on a local installation or on a staging system that is password-protected, which prevents Mailchimp from checking the connection.

In this case, you can disable password protection, save again, and see if the error message reappears. If you then reactivate password protection, subscribers who unsubscribe will not be synchronized.