However, if the issue persists, I'd suggest disconnecting and reconnecting the MTD feature since you've submitted returns without any problems before. It helps refresh the connection between the HRMC and QBO. I'll guide you how.
To disable the MTD connection:
Go to the Gear (Settings) icon at the upper right.
Select Account and settings under Your Company.
Go to Advanced from the left menu.
Choose Making Tax Digital.
Click the Learn how to disconnect button.
You can also click Turn off MTD in the Edit VAT settings window to disconnect it.
Moreover, you can run any VAT reports in the software. One of them is the VAT Detail report. It helps you view the transactions that are included in each box on the VAT return. Just go to the Reports or Taxes menu to complete this task.
I'm always here to help if you need anything else. Enjoy the rest of your day, @rees-faidre.