Fatal error: Uncaught Error: Interface "Symfony\Component\Serializer\Encoder\EncoderInterface" not found in /home/www/drupal8/test/core/modules/serialization/tests/serialization_test/src/SerializationTestEncoder.php:7 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/serialization/tests/serialization_test/src/SerializationTestEncoder.php on line 7