Successful redirect but no apparent authorization [#3636582]

Problem/Motivation

Trying to connect a local DDEV site to Mailchimp before doing this on a live site. Authorization seems to work on the Mailchimp side but after redirecting to /admin/config/services/mailchimp/oauth nothing has changed there, and reliant modules like Webform Mailchimp do not have access to the API>

I do see in the console "Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://drupal-oauth.mailchimp.com/status?domain=example.ddev.site&temp_...."

With the reason given "CORS request did not succeed"

Steps to reproduce

From a local DDEV site, initiate Oauth2 connection from /admin/config/services/mailchimp/oauth

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Issue fork mailchimp-3636582

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3636582-successful-redirect-but changes, plain diff MR !131
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