The cause:

In this case, the whole export fails. The reason is that, for Pleo’s integration with Fortnox to work properly, the verification series in Fortnox needs to be marked as manual.

The solution:

In Fortnox, go to Settings (Inställningar) - Bookkeeping (Bokföring) - verification series (Verifikationsserier). Next to the Pleo series name, tick the box in the manual column (Manuell Kontering).