I had the same problem. I had not indicated the Terms of Payment when I initially created the invoice. Even after I fixed it in the Customer tab, it still showed that invoice as Past Due. I found a quick work-around - this is not a fix, but it worked for me. After fixing the terms of payment under the Client, I opened the invoice and made a copy of it (keep in mind it will give your copy a new invoice number). Then I clicked Save and Close. Then I opened the old invoice that kept showing Past Due and I deleted it. Now I only have the new one and it looks fine.