I had exactly this problem and tried all of the other fixes such as disconnecting then reconnecting non of which worked.
In the end, I rang Quickbooks who solved the problem in a couple of minutes.
In the Taxes section, click Edit VAT on the top right and check your VAT number does NOT have any spaces in it. If it does, remove the spaces and it will hopefully fix the connection issue.
It worked for me and I managed to submit my VAT return at long last
