It's solved now in CVS (1.3 & 1.4dev)
Just see Bug 1702 for more info.
Ciao
PS: That script is an outdated and basic version of the official "scheduled backups" feature, so I highly recommend you to avoid using it. Probably we'll delete it in future releases...
Backup and restore
Error when trying to run an ad-hoc full backup using /backup/try.php
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