Find out when that nominal last had a zero opening balance. View the nominal ledger and change the from date until you see the opening balance with a value.
Review the transactions within that date period, double click on a posting line to open the transaction info sheet with the double entry details.
Check your Retained Profit nominal to spot any differences there within the date range selected. Click the DR and CR of the debit and credit columns to sort them into value order.
It is usually the case that a journal entry needs to be posted between the profit and loss nominal to bring it to zero, and the Retained Profit nominal.
If you are still having issues, please raise a new case online and reference the title of this article.