The cause:
One (or more) expenses in Pleo has a tag that reflects an inactive project in Fortnox.
There are two possible solutions:
In Pleo, change the tag on the affected expense(s).
Or
In Fortnox, change the project accounting period:
Click "Register".
Click "projects" (Projekt).
Find the relevant project under “completed” (Avslutade):
Change either the end date or the status of the project:
Click "save" (Spara).