I had the same issue here and dug a little deeper, hoping to avoid having to edit the plugin. For me, as with @andyford, the issue only occurred on my Media Temple environment. Their infrastructure was confusing the WordPress is_ssl() function, which was firing false at the time the asset urls were being generated (from functions used in ACF like content_url() and plugins_url()).
Good news is, there is a solution in the WordPress codex. Just add the following to your wp-config.php file:
if (isset($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https') $_SERVER['HTTPS'] = 'on';
This solved my mixed-content issues without having to edit the plugin.
