The cause:
One or more Pleo expenses have a tag that either 1) never existed in Fortnox (an error was made when it was created manually in Pleo) or 2) it used to exist in Fortnox, but was deleted.
The solution:
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).