Linux Perform MySQL Backup to time-based path from Webmin
MySQL Database Server > Module Config > Chose Yes on "Do strftime substitution of backup destinations?"
It work for me! :)
You can use dynamic path, but before you must enable it in module config:
System > Filesystem Backup > Module Config > Do strftime substitution of backup destinations? > Yes
(If you are not sure about placeholders, just click on the text "Do strftime substitution of backup destinations?" in the config and help will show you.)
I had the same problem and solved like this :
From Server select MySQL Database Server. Go to Module config (top left), and select Do strftime substitution of backup destinations? to Yes.
I use db_%d-%m-%Y.sql format for backups. (db_13-03-2013.sql)