Fatal error: Uncaught Error: Class "Drupal\serialization\Normalizer\ComplexDataNormalizer" not found in /home/www/drupal8/test/core/modules/serialization/src/Normalizer/FieldItemNormalizer.php:12 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/serialization/src/Normalizer/FieldItemNormalizer.php on line 12