Fatal error: Uncaught Error: Class "Symfony\Component\Validator\ConstraintValidator" not found in /home/www/drupal8/test/core/modules/user/src/Plugin/Validation/Constraint/UserNameConstraintValidator.php:11 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/user/src/Plugin/Validation/Constraint/UserNameConstraintValidator.php on line 11