Issue reproduced and ticket updated with details:
https://issues.freepbx.org/browse/FREEPBX-20017
You can wait for the bug to be addressed so you can upgrade, or if you want to just lose your existing backup jobs and recreate them in 15 you can do the following:
fwconsole ma downloadinstall backup --tag 14.0.10.7
fwconsole ma delete backup
fwconsole ma downloadinstall backup
edit - fixed by upgrading to v. 15.0.8.21