I found two spots that can cause the security prompt for mixed content esp in IE. 1. events-calendar-pro.php line 49 is still using WP_PLUGIN_URL instead of plugins_url. 2. tribe-filters.class.php line 412 when your calling the jquery-ui.css it uses http, i just changed it to https for now but i’m sure you could add a check for https or not and call the correct one or just include that file.