The software is version 8.0.8, "test and correct files" reports no problems with the files/data.

Last year I reconciled the account "by hand", and adjusted the beginning balance to the correct balance. The account was in balance for a short while, and then the "unreconciled difference" appeared again.

Run a report for Transactions by Account on the affected bank journal and match it to the transactions on the Bank Statement. Any item not on the Bank Statement should not be "cleared" in FN.

Verify that the client is not posting transactions in a general journal for the bank account. This will affect the balance of the bank account but will not show up in the transaction activity screen for that journal. See "Regarding Step 4" in the post Bank Reconciliation.

Verify that the client is using the Bank Statement balance when prompted during reconciliation and not the FN balance.

See the FN onscreen Help index item Bank Statement: Reconciling for further information on reconciliation steps.