Hi there, 100seanby. 

I've got some steps we can perform to fix the error message when filing the VAT report in QuickBooks Online (QBO). 

Issues like this can occur when your browser stores too much historical data. You can get rid of them in a few steps. Let's start by signing in to your QuickBooks Online account in a private/incognito window.

Here are the keyboard shortcuts:

• Google Chrome: Ctrl + Shift + N
• Mozilla Firefox: Ctrl + Shift + P
• Microsoft Edge: Ctrl + Shift + P
• Safari: Command + Shift + N

Once in, try to file a VAT report.  If it works, go back to your regular browser and clear its cache. This is done to speed up the process the next time you do a transaction in QuickBooks Online. Also, an alternative way is to use other compatible browsers.

I've added this article to learn more about filing taxes in QuickBooks:  File your VAT return and record tax payment in QuickBooks Online.

Keep your post coming if you have any other questions related to VAT. I'm always here to help. Have a great day!