Usually if the account was reconciled previously and on the next reconcile the beginning balance is off it means you have either deleted or changed the amount of an already reconciled transaction, or you have marked a transaction that shouldn't be reconciled as reconciled.

Look at your register for that account.The transactions with a dark checkmark are reconciled. Are there any "off in the future" that are marked as reconciled?  You may have clicked one of them by accident.