I have discovered what the problem is - MySQL does not allow a "GROUP BY ... WITH ROLLUP" to be used with "ORDER BY ...". The solution is to edit the getdata() method inside the 'dml.mysql.class.inc' file in order to change
if (!empty($this->sql_groupby)) {
$group_str = "GROUP BY $this->sql_groupby";
} else {
$group_str = NULL;
} // if
to the following:
if (!empty($this->sql_groupby)) {
$group_str = "GROUP BY $this->sql_groupby";
if (eregi(" WITH ROLLUP", $this->sql_groupby)) {
$this->sql_orderby = null;
} // if
} else {
$group_str = NULL;
} // if
This will cause the ORDER BY clause to be ommitted if the GROUP BY clause contains "WITH ROLLUP".
This change has been tested with version 1.24.0 and will be included in version 1.25.0