Carbon Fields http Mixed Content

Use HTTPS everywhere:

/* wp-config.php */
define( 'WP_HOME',    'https://domen.com' );
define( 'WP_SITEURL', 'https://domen.com' );
define( 'FORCE_SSL_ADMIN', true );

Force REST to https:

add_filter( 'rest_url', function( $url ) {
	return str_replace( 'http://', 'https://', $url );
} );

Clear caches - mixed-content error gone.