WP_PLUGIN_URL equivalent

WordPress is expecting http, but you’re visiting with https which is causing the mixed content error. Setting your site and address URLs to use https should solve your problem.

If you can’t use https in the admin, then you’ll need to try ways to tell WordPress to use https only on the frontend. Try something like this (untested):

add_filter( 'site_url', static function ( $url ) {
    if ( is_admin() ) {
        return $url;
    }

    return str_replace( 'http://', 'https://', $url );
} );