Set SERVER_PORT to 80

wp-config.php.template

@@ -57,6 +57,7 @@ define('WP_DEBUG_LOG', false);
 // If WordPress is behind reverse proxy which proxies https to http
 if (!empty($_SERVER['HTTP_X_FORWARDED_FOR'])) {
     $_SERVER['HTTP_HOST'] = $_SERVER['HTTP_X_FORWARDED_HOST'];
+    $_SERVER['SERVER_PORT'] = '80'; // pretend apache is running on port 80. plugins like rsvp use this
 
     if ($_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https')
         $_SERVER['HTTPS']='on';