Definitely a bug, seems that after the first successful backup unit will no longer get past the system file.
UPDATE, your backups will complete if you do not change the name. Leave it as the date or default name If this does not work for you the only other thing I have done since it was rebooting was to install/update busybox. This may also work for those who are restoring backups and getting the reboot...did you change the file name? Whatever I am sure it is just a temp bug that will be fixed soon as well as the layout.