The cause:
One or more Pleo expenses have a tag (reflecting a cost centre 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 cost centres. 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:
In Fortnox, update the affected account/category to allow for cost centres:
Click "Register".
Click "chart of accounts" (kontoplan).
Click on the affected account (konto).
Under "cost centres" (kostnadsställe), change the option from "not allowed" (Ej tillåtet) to either "allowed" (Tillåtet) or "mandatory" (Obligatoriskt):
Click "save" (spara).