I have created a backup to an ftp server. The backup looks like it runs without any errors and creates the file but the file is 0 kb in size. Running Freepbx distro version 2.210.62-1 and asterisk version 1.8.19.1.
to verify that you are indeed backing up what you want. i suspect you have not setup the ftp site correctly. i know it works because we use it every night.