Error: Wrong user: my userid is 81, instead of 101(backuppc)

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/webapps/backuppc/2.1.2-r1/hostroot/cgi-bin//BackupPC_Admin and look at the documentation.