I am have the same exact issue as the original poster.  I have checked EVERYTHING you mentioned, none of it fixes the problem.  None of the customers/items/accounts that are inactive have been changed within the past several months.  This problem just started late last week.  I think it's related to the awful update that Quickbooks did to their online invoices, but I don't know how to fix it. I cannot even create an invoice. This is a huge problem that needs to be fixed immediately.  

Since I've checked everything on my end that you mentioned, and none of that was the problem, what can I do?