I suggest to investigate further. There are few things I can suggest you to try:
1) One other way to look customer balances is through Customers window.
- Go to View -> Customers.
- Check the 'Balance' column for the customer in the table.
This balance should be same as the "unpaid accounts report" balance for the customer.
2) Now, find out the Opening Balance of the customer.
- Double-click on the customer from the list, and go to 'Info' tab.
If Opening Balance is big, this may be the reason for the huge figure.
- Adjust the opening balance and click 'OK'
3) Finally, look into customer statement.
- From the 'Customers' window, right-click on the customer -> Select 'Statement -> Preview, Print or Save Statement'.
- Select the desired time period, and click 'OK'
It should show you all invoices, payments, opening balance, and current balance for the selected period.
Hope this helps!