The cause:
One or more Pleo expenses fall on a date that's been automatically locked in Fortnox.
The solution:
If the Pleo expense is a card expense:
In Fortnox, go to Settings (Inställningar).
Go to Accounting (Bokföring).
Click on Locking of period (Låsning av perioden).
Either toggle from "yes" to "no", or adjust the dates of the lock period.
If the Pleo expense is a Pocket expense:
In Pleo, change the date of the Pocket expense so that it falls within an open accounting period:
