I would narrow down the accounts and date range to see where and when the problem first occurs. Then look for a transaction on that date.
Back up your data just in case, then delete and re-enter the problem transaction.Hopefully that will fix it.
Is the validation problem new, or has it been going on for a long time? If you have been validating regularly and the validation problem is new, you could try restoring a recent backup to see if that fixes it.