LogicException: Settings can not be serialized. This probably means you are serializing an object that has an indirect reference to the Settings object. Adjust your code so that is not necessary. in Drupal\Core\Site\Settings->__sleep() (line 90 of /app/web/core/lib/Drupal/Core/Site/Settings.php).
Solution:
Add dependencySerializationtrait to the checkout event subscriber.
LogicException: Settings can not be serialized. This probably means you are serializing an object that has an indirect reference to the Settings object. Adjust your code so that is not necessary. in Drupal\Core\Site\Settings->__sleep() (line 90 of /app/web/core/lib/Drupal/Core/Site/Settings.php).
Solution:
Add dependencySerializationtrait to the checkout event subscriber.