Опечатка:
server (optional) losetup /dev/loop0 /mnt/bc/backup.img mkdir /mnt/backup_server cryptsetup luksOpen /dev/nbd0 backup_server
server (optional) losetup /dev/loop1 /mnt/bc/backup.img mkdir /mnt/backup_server cryptsetup luksOpen /dev/loop1 backup_server -- Sincerely, Nicholas