The FTP backup feature is not working correctly on FreeBSD. This is the output of trying to execute an FTP backup.
tar: Option --rsh-command=/usr/local/etc/webmin/fsdump/ftp.pl is not supported
Usage:
List: tar -tf <archive-filename>
Extract: tar -xf <archive-filename>
Create: tar -cf <archive-filename> [filenames...]
Help: tar --help</archive-filename></archive-filename></archive-filename>
You may need to install GNU tar rather than the default BSD tar command.