The cause:
One or more Pleo expenses has a category that does not exist in Fortnox (i.e, the respective "konto" is not active). It could be that it did exist, but was deactivated.
There are two alternative solutions:
In Pleo, change the category of the affected expense.
Or:
In Fortnox, re-activate the category/account:
Click on "Register".
Click "chart of accounts" (Kontoplan).
Click on “inactive” (Inaktiva) to get a list of archived accounts:
Find the affected account (Konto).
Under "active" (Aktikt), switch from "no" to "yes":
Click "save" (spara).