The cause:
One or more Pleo expenses have a tag that's linked to a de-activated cost centre in Fortnox.
There are two alternative solutions. Either:
Re-active the cost centre in Fortnox:
Click "Register".
Click "cost centres" (Kostnadsställe).
Find the relevant cost centre under the "deactivated" (Inaktiva) list:
Under the "active" option, change the answer from "no" to "yes", then click "save":
Or:
In Pleo, change the tag on the affected expense(s).