I forget the Internet also has weekends
Shell command didn't work; "Unable to find file or command."
Deleted the file, then did the re-write, backed up "successfully" the first time, fails at later attempts.
I do believe that the "successful" backup actually has no effect. I remember seeing these messages before wiping my system through TWRP 2.2.2.1 Recovery, but would later find out that the settings actually failed.