The message :
This ownCloud instance is currently being updated, which may take a while.
This page will refresh itself when the ownCloud instance is available again.
Contact your system administrator if this message persists or appeared unexpectedly.
Thank you for your patience.
Solution :
If you have a console access, go to your owncloud directory (eg. /var/www/owncloud/)
Execute the following command :
# su -c "php occ upgrade" www-data
If you don’t have a console access, the only bad solution is to edit the configuration file config/config.php and to change the value of maintenance to false
'maintenance' => true,
Become :
'maintenance' => false,