The cause:
One or more receipts are causing the export to fail.
The solution:
Export from Pleo without the attached receipts:
Download the receipt(s) in question and save them on your device.
In Pleo, remove the receipt(s) from the affected expense(s).
Export to Fortnox.
Manually upload the receipts in Fortnox.
You can always re-add the receipts to the expenses in Pleo (after exporting). That way, the cardholder/expense owner won't get notified about missing receipts.