It seems like an invoice got created with a wrong date. Causing the problem. I would turn off interest on this client. Then make sure the current  balance shows as $0. Run a bill and make sure you are set to include clients with $0 balance. If all that is good, then you should be able to turn interest back on without future issues.

Thanks for the info.  I went and checked my invoices and payments and they're all dated correctly and applied correctly.  The aged A/R listing shows no old balances for the client, but even after turning the interest off the invoice itself shows an opening balance of $900. If I try to enter a new payment, the invoice details of the old invoice show on the screen but it shows that payments were already applied and the outstanding amount of the invoice is zero.  So I'm left with a bogus opening balance on the new invoice.  Any other ideas?

Try generating a statement for the client. Show all details (options). Beyond that, probably need some "hands on".

Thanks, I'll call tech support.  I went to "generate statements" and the client wasn't even listed to be selected.  Sigh.  Thanks for trying.