The command to set a user's quota is not defined
This may be a bug.
When I go to "Module Config" for Virtualmin, then go to "Use external commands to get and set quotas?" and click "Yes", leaving all the "use standard programs" radio buttons set and then click "Save" and then "Recheck and Refresh Configuration" I get the error message "External quota commands are enabled, but the command to set a user's quota is not defined or does not exist"
This raises an alarm with me, because before I upgraded to the latest version of Webmin (1.390), this was working.