The sync directory is defined in $settings and not $config_directories

Author: radimklaska

docroot/sites/default/settings/includes.settings.php

@@ -15,7 +15,7 @@
 /**
  * For custom installation
  */
-$config_directories['sync'] = "../config/default";
+$settings['config_sync_directory'] = "../config/default";
 
 /**
  * Set content directory for default_content_deploy.