The cause:
The contra account that's been set up for Pleo has not been set as a bank account type in Quickbooks, and so the full export fails.
The solution:
In Quickbooks, go to your chart of accounts.
Find the contra account(s) that you've set up for Pleo. If you're not using Pocket, you'll only have one Pleo contra account. If you are using Pocket, you should have more than one.
Click on the account and change the account type to "Cash at bank and in hand", and the detail type to "Current".
Click "Save and Close".