Found this after enabling xmlsitemap in D8 ver. beta 9
Drupal\Core\Config\ImmutableConfigException: Can not set values on immutable configuration xmlsitemap_engines.settings:engines. Use \Drupal\Core\Config\ConfigFactoryInterface::getEditable() to retrieve a mutable configuration object in Drupal\Core\Config\ImmutableConfig->set() (line 34 of /Users/tasya/Sites/xmlsitemap/core/lib/Drupal/Core/Config/ImmutableConfig.php).