Update to Drupal 8.2.6. For more information, see https://www.drupal.org/project/drupal/releases/8.2.6

vendor/symfony/validator/Constraints/AbstractComparisonValidator.php

@@ -44,7 +44,7 @@ abstract class AbstractComparisonValidator extends ConstraintValidator
         // the DateTime constructor:
         // http://php.net/manual/en/datetime.formats.php
         if (is_string($comparedValue)) {
-            if ($value instanceof \DatetimeImmutable) {
+            if ($value instanceof \DateTimeImmutable) {
                 // If $value is immutable, convert the compared value to a
                 // DateTimeImmutable too
                 $comparedValue = new \DatetimeImmutable($comparedValue);