The cause:
One or more Pleo expenses have a tag that reflects a deactivated cost centre in Fortnox.
There are two alternative solutions:
Update the tag in Pleo
In Pleo, it's a good idea to re-import your tags from Fortnox to make sure that your list is up to date. When you re-import, Pleo will automatically archive any older tags that no longer exist in Fortnox.
Once you've re-imported your tags, update the tag on the affected Pleo expense(s).
OR
Re-activate the cost centre in Fortnox:
Click "register".
Click "Kostnadsställen".
Go to the "inactive" (Inaktiva) list.
Click on the relevant cost centre.
Under "active" (Aktivt), switch from "no" to "yes":
Click "save" (spara).