Route "update.manual_status" does not exist

Hello,

I am trying to learn a bit about Drupal 8 and I install the latest version (8.0.3).

I try to install the "Update manager" through the "Extension" menu as it was stated in the "status report" and I get an error :
Symfony\Component\Routing\Exception\RouteNotFoundException: Route "update.manual_status" does not exist. in Drupal\Core\Routing\RouteProvider->getRouteByName() (line 191 of core/lib/Drupal/Core/Routing/RouteProvider.php).

All admin actions are impossible as I get always that message ....... the site content is still usable.

How to "repair" the admin functions ?

Regards

JP P