Can't get cookie free domain working for my WordPress page

I'm trying hard to get my cookie free domain working for my WordPress installation. I've followed a lot of tutorials but I still get no content over the new cookie free subdomain. These are my steps: 1. I've created a new subdomain static.my-page.de in my Plesk panel and set my wp-content folder as document root for this subdomain 2. I've logged in into my domain provider account, added a subdomain under my domain my-page.de and created an A-Record to my Plesk server (entered the IP like for all other subdomains) 3. After this I've enabled SSL for the new subdomain and selected my wildcard certificate After this I've did the needed changes in my WordPress wp-config.php file: define( 'WP_CONTENT_URL', 'https://static.my-page.de ' ); define( 'COOKIE_DOMAIN', 'https://www.my-page.de ' ); Finally I've changed all my post_content entries in my DB to my new static subdomain. Normally I should get all my wp-content folder content via my static non-cookie subdomain but the page design is completely broken and the console full of errors like this: (index):1830 GET https://static.my-page.de/plugins/ultimate-member/assets/js/um-members.min.js net::ERR_NAME_NOT_RESOLVED What did I do wrong? I've gone over everything 100 times and no luck...