Copy the backup file to your Desktop, rename it to backup.zip. Then you can enter following command in Terminal:
zip -F ~/Desktop/backup.zip --out ~/Desktop/backup-fixed.zip
If that is not able to repair the file, try with this:
zip -FF ~/Desktop/backup.zip --out ~/Desktop/backup-fixed.zip
If this works, rename the file backup-fixed.zip to backup-fixed.st3, copy it to your iPad via Airdrop and try to restore it.