On PHP 7.2 we cannot start session until it has been configured by Drupal.

extern/rest.php

@@ -31,7 +31,6 @@ $config = CRM_Core_Config::singleton();
 if (defined('PANTHEON_ENVIRONMENT')) {
   ini_set('session.save_handler', 'files');
 }
-session_start();
 $rest = new CRM_Utils_REST();
 
 // Json-appropriate header will be set by CRM_Utils_Rest