Vat calculation is wrong

Welcome to the Community, @patriclguntert.

There may be transactions that were either edited or deleted. This is why the calculation of your VAT was off. If that's the case, let's check the Audit Log report in QuickBooks Online (QBO) to verify.

Here's how:

1. Go to the Gear icon at the top right to get to the Audit Log.
2. Click the Filter button, then select the Show only these events radio button in the Events section.
3. Check the box for Transactions.
4. Hit the Apply button.

For more information, please refer to this article: Use the audit log in QuickBooks Online.

Just in case, I'll add these articles for future reference:

• Set up VAT in QuickBooks.
• Common VAT Codes.
• UK Flat Rate Scheme: How does it work?

Please let me know if you have any other issues or concerns in the comment below. I'm more than willing to help. Have a great weekend!