I thought I'd post something here in case it happens to others. We updated our Quickbooks Pro 2016 to the most recent update and that broke the integration between Quotewerks and Quickbooks. As soon as I'd try to push an invoice to QB, Quickbooks would crash looking for a lock file. Spent a week dealing with it then got the notice about the 5.3 Build 4 update. On a whim I loaded it and the problem went away. I believe QB has put in place some things to manage GDPR - conjecture on my part but happy to know the Build 4 fixed it. I was not looking foward to manually entering dozens of invoices.