If you do not like running the upgrade in the browser after the software upgrade, you can it on the command line as well:
sudo su - apache2 cd $WEBROOT/owncloud ./occ upgrade
If due to an error the application remains in maintenance mode after the installation, this can be overwritten by the parameter 'maintenance' in
<root>/config/config.php. This must be set to false in order to disable maintenance mode.
Cannot create user
When (trying) to create a new user, an error appears:
The username is already being used.
The usual cause for this is an enabled authentication app, like WEBDAV authentication. The point of conflict is the crossing of local and remote user accounts. As long as only local user accounts are being used, there is not need to have remote authentication apps being enabled. If local and remote accounts shall be mixed, disable the app, add the user and re-enable the app afterwards.