Hi Archie, thanks for dropping by
In most cases where users have been submitting successfully previously and no details have changed on the Gov Gateway account (such as login info), the cause of the message is due to expiration of the authorisation period.
As part of HMRC's security requirements, authorisation needs to be provided every 18 months. We'll alert you in product that you need to take action, but if you don't see this, you can still re-authorise by disconnecting MTD in QuickBooks (and then reconnecting).
If you continue to see any error message after this, please get back in touch below and we'll pick it back up with you.
