The new setting will allow to host the temporary backup files
into a specific target directory. Defaults to '$CFG->tempdir/backup'.
Calling make_backup_temp_directory() checks that the required sub-directory
will be properly created under the new target directory.
It was not previously possible to have a section called 0 because of
bugs in the standard course formats, but we hit this with the OU course
format. You got an exception because backup settings tested to see if
their lable was empty, which means a section name of '0' was fatal.
Should work now.
Matteo Scaramuccia