The cause:
One or more Pleo expenses have a tag (reflecting a project in Fortnox) that conflicts with a category (reflecting an "account" in Fortnox). It conflicts because the selected category/account is not allowed to be connected to projects. This is something that's been set up in Fortnox.
There are two alternative solutions:
In Pleo, change the tag (or category) on the affected expense(s).
Or:
Change the account in Fortnox so that it's allowed to have associated projects:
Go to Register.
Click chart of accounts (Kontoplan).
Click on the relevant account (konto).
Under projects (Projekt), change the selection from "not allowed" to allowed (Tillåtet):
Click save (Spara).