Hi DrMouse, thanks for joining this thread. 

Please note that the authorisation process in QuickBooks will accept PAYE credentials as well as VAT ones, but the credentials for your PAYE and VAT accounts with HMRC will be different. Please ensure that you're using the correct credentials when connecting in QuickBooks. 

If you're still unable to submit after checking this, contact us directly by chat or on 0808 234 5337. We'll be able to take some account details to check the specific error being returned from HMRC. :)