You'll want to check if there are any updates from the HMRC connection, Ian.

Since QuickBooks acts as the submission platform, the error suggests that HMRC's online service is currently too busy to accept the VAT return. I recommend visiting the HMRC website and service tracker to look for notices about system maintenance or service interruptions, as this can help confirm if the issue is affecting others as well. 

If official downtime is confirmed or suspected, the simplest solution is to wait and try submitting the return again tomorrow. You can also refer to the article and perform the suggested fixes in accordance to the specific error message you've encountered: Fix MTD (Making Tax Digital) issues in QuickBooks (VAT and ITSA).

If the issue persists or if you are near the deadline, you may need to contact HMRC's help desk via the GOV.UK site for specific guidance on your return.

Please reply to this thread if you have additional questions or need further assistance.