Hi!
Thank you for the details, it helps a lot!
The issue is, that the site URL might be set to a non-https version somewhere, either on the back-end, or within the wp-config.php file, as some stylesheets on the site front-end are returned with the non-https urls (causing the issue).
Make sure to check on your site back-end, that the home URLs are set correctly there. If that is correct, then it might be overwritten somewhere in the database. In that case, placing this code to the wp-config.php file will resolve that as well:
[php]define(‘WP_SITEURL’, ‘https://www.yoururl.com/’);[/php]
Then clearing your site cache, and browser cache should then get rid off all the https errors, and display the plugins correctly.