I did backup but the content of sql file was php errors, saying, upload_tmp_dir setting is "/var/lib/php/tmp" and backup.php tried to open "/var/lib/php/tmppostfixadmin-20090730-19251.sql", witch is out of open_basedir setting "/var/lib/php/tmp".
ini_get('upload_tmp_dir') doesn't have a last slash in my environment, but backup.php just attaches a file name to the upload_tmp_dir without slash.
I just fixed to work in 57 to 59 like below.
$path = (ini_get('upload_tmp_dir') != '') ? ini_get('upload_tmp_dir') : '/tmp'; // '/tmp/' -> '/tmp'
$filename = "postfixadmin-" . date ("Ymd") . "-" . getmypid() . ".sql";
$backup = $path . DIRECTORY_SEPARATOR . $filename; // added a DIRECTORY_SEPARATOR
I hope this bugfix will be applied in the next release.