500 error (Guzzle/cURL) when trying to authenticate domain name

The MailchimpAdminOauthSettingsForm::getMiddlewareUrl() method is retrieving the 'mailchimp.settings' configuration and returning the 'oauth_middleware_url' key. When I look at the config in the database and the exported configuration file, there isn't a key for 'oauth_middleware_url'.

The documentation doesn't detail how to set this value. Can you assist?

Disregard,
I figured it out. Somewhere in my process I failed to update the database drush updb after updating the module code.

I ran into the same CURL error "cURL error 3"

Somehow the latest Mailchimp update in mailchimp.install was never executed.

As a quick fix I added the latest update to a custom module, after running this, the CURL error went away.

If you have any problems make sure to check row "mailchimp.settings" in the "Config" table, as @jumpsuitgreen states, "oauth_middleware_url" must be present.

Make sure to run below:

function mailchimp_update_8004(&$sandbox) {
  $config = \Drupal::configFactory()->getEditable('mailchimp.settings');
  $config->set('oauth_middleware_url', 'https://drupal-mailchimp-oauth.uw.r.appspot.com');
  $config->save();
}