The cause:
This message comes up when a Pleo expense includes a tax code that isn't valid in Datev. The end of the message always specified which code is affected (e.g., the value '09' in the above image). The rules for tax codes in Datev are:
1-3 digits long
They cannot start with 0
The solution:
Please check your tax codes in Pleo under Settings → Accounting → Setup to make sure that everything matches Datev's rules.
Please add an updated tax code to the affected Pleo expense(s) before you re-do the export.