This script needs to run as the user specified in $Conf{BackupPCUser}, which is set to backuppc.
This is an installation problem. If you are using mod_perl then it appears that Apache is not running as user backuppc. If you are not using mod_perl, then most like setuid is not working properly on BackupPC_Admin. Check the permissions on /usr/share/BackupPC/sbin//BackupPC_Admin and look at the documentation.