In Administration >> Configuration >> Variables >> Operating System is an entry for pathtodu. This should specify that GNU du is needed since du is called with --apparent-size, a GNUism. Otherwise, the error logs will contain lines similar to:
/usr/bin/du: illegal option – apparent-size
usage: du [-a] [-d] [-h|-k] [-r] [-o|-s] [-H|-L] [file ...]

The page in question is lang/en_utf8/admin.php