Does phpMyAdmin only works with HTTPS?
No, phpMyAdmin doesn't require using HTTPS. Some server configurations seem to be experiencing this problem, which is reported at the phpMyAdmin issue tracker at https://github.com/phpmyadmin/phpmyadmin/issues/14184.

For me, trying to log in a second time works, for others there are add-ons such as Katalon Recorder (Selenium IDE for Chrome)  or uBlock Origin preventing the file /js/vendor/js.cookie.js?v=4.8.2 (or whatever your version is) from loading.

Finally, there can be some cookies "stuck" between secure and insecure mode; you may be able to clear those directly through your browser (phpMyAdmin cookies start with "pma").