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,