The cause:
Some of our customers use a single Exact login for multiple entities or "administrations". If you accidentally connect to the wrong administration when setting up the integration, then the export will fail.
The solution:
In the Accounting Settings page in Pleo, disconnect from Exact Online:
Reconnect the Exact Online integration. When reconnecting, choose the correct Exact entity (in which you've already set up a Pleo-dedicated bank journal account, purchase journal, etc.).