I had the same problem before, which was resolved with customer service. It has happened again and what worked for me is to click the gear icon on the top right, then click on payroll settings, then scroll down to export at the bottom, click on the pencil sign to edit, then click on sign in at the top to connect the Quickbooks Online to the Payroll account. Use the Quickbooks Online login credentials and use a Chrome based browser instead of Firefox for the sign in process. And voila!
