Hi Zappy,

Thanks for posting in the Community!

There could be an edited invoice/sales form number when this behavior is encountered. Let's run an invoice list report in your QuickBooks Online to verify if the invoice numbers used. Here's how:

1. Click on the Reports tab.
2. On the search box type in Invoice List.
3. Filter the Report period to All Dates and click on Run report.
4. Verify if there are no duplicate invoice numbers.

I recommend also checking your Audit log to verify if there were changes made to an invoice/sales forms. Here's how:

1. Click on the Settings icon.
2. Click on Audit Log.
3. Filter from User, Dates or Event.

The audit log shows you the most transaction edited or most recent events. Click on View to see the History made.

Furthermore, I can see the previous contact you've made has been escalated to our senior support for further investigation. 

Drop by again in the Community if you need more help with QuickBooks Online.