UUIDs are now validated

Core's uuid field type -- which is automatically used to store content entity UUIDs -- now validates that the value it holds is, in fact, a valid UUID.

Previously, this was not the case; only the value's length was validated. So as long as it fit within a certain size, you could have an invalid UUID like this-is-my-entity. As of #3559874: The UUID data type should validate that its value is an actual UUID, such a value will no longer pass validation.

Most people are unlikely to run into this, but it may cause any entities with invalid UUIDs to fail validation. If that happens to you, you must change the entity's UUID to something valid:

$entity->set('uuid', \Drupal::service('uuid')->generate())->save();