After standard installation, straight out of the box the status report says "Update notifications are not enabled."
However the "Update Manager" checkbox on the module admin page is checked.
Pressing its config link gives error "Error The website has encountered an error. Please try again later." on oherwise blank page.
More usefully (I hope), the error log shows
InvalidArgumentException: Class "\Drupal\update\UpdateSettingsForm" does not exist. in Drupal\Core\DependencyInjection\ClassResolver->getInstanceFromDefinition() (line 29 of /home/adrian/Drupaldev/testserver/d8t1/core/lib/Drupal/Core/DependencyInjection/ClassResolver.php).
This is reproducible
Workaround :
The checkbox cannot be unchecked, but the module can be uninstalled from the uninstall tab, and then re-installed. That fixes the problem.
System:
Desktop running OpenSuse 13.1, php 5.4.20 and MariaDB 5.5.33 and Apache 2.4.10