I think I’ve specifically tested this and due to a CORS issue if you’ve visited your site from the original URL, which causes the resources to be cached, then visit it from your new custom domain URL, then the browser would improperly remember the CORS response from the cache, causing a failure.
Clearing your browser cache should resolve this issue. If it does not, remove your password and add it again, then clear your browser cache. This should force our server-side cache to clear.
As a visitor, if they don’t visit your site using the old URL, they should never run into this issue.