Automated backup doesn`t work

The automated backup doesn`t work in my new 2.5.1 installation.

I´ve enabled the automated backup in courses/backups/automated... and left everything else by the default values. 

In the server console I typed

crontab -e

and wrote: 

*/30 * * * * wget -q -O /dev/null http://www.xyz.at/moodle2013/admin/cron.php

But there are no backups in the courses and there is no log in Reports/backups.

I`ve changed the backup_auto_destination to \backup where wwwrun is the owner and www the group. No success.

Anyone any idea?