Does cron.php belong to www-data?
also,
"00 2 * * * root sudo -u www-data php -f /var/www/html/owncloud/cron.php"
should be
*/15 * * * php -f /var/www/html/owncloud/cron.php
for the user www-data.
Try calling the crontab like this:
“sudo -u www-data crontab -e” and then paste the above line inside. I think you’re using the the wrong crontab
Greetings,
Maulwurf