Fatal error: Uncaught Error: Class "Symfony\Component\Serializer\Encoder\JsonEncoder" not found in /home/www/drupal8/test/core/modules/serialization/src/Encoder/JsonEncoder.php:18 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/serialization/src/Encoder/JsonEncoder.php on line 18