The cause:
A Pleo expense is missing a category that reflects an active expense account in Quickbooks. It's not possible to export expenses without a relevant expense account, so the export fails. It might be that you've included a category, but it's not active in Quickbooks. If a category is missing entirely, you might get the message "detail must not be null", in which case the below solution also applies.
The solution:
Make sure that you've synced your Pleo categories with Quickbooks.
Add a category to the affected Pleo expense.