Database Backup and Restore from the admin menu is broke
Chealer9 added this to the branch-1-8: Warning : Backups are known not to work since some time now. Don't rely on them. Chealer9 20031117
But I think this also extends to the 1.7.5
If you need to backup the database the best way is with either phpMyAdmin downloadable from http://sourceforge.net/projects/phpmyadmin or just using the mysqldump command directly at console.
mysqldump -u dbuser -p dbname > dump.sql
will ask for the password and backup dbname to dump.sql
mysql -u dbuser -p dbname < dump.sql
will ask for the password