Wallet loads from Pleo can also be exported to Datev, in which case they show up as credits which are then offset against Pleo expenses (debits). For this reason, the account in Datev needs to be able to go into minus with the total balance. When Datev isn’t set up this way, the above error message appears in Pleo.
Change the account details in the relevant Datev account. To do so, click “Optionen” (options) in the top right, then click then “Kassenstammdaten verwalten” (manage account data).
Next, click "deaktivieren” (deactivate). Please note that you might need special permission to be able to change this in Datev.
If it's not possible to change the above setting (i.e., the button is greyed out), then the account (Kasse) should be deleted and re-created with the negative account balance (Kassenminusprüfung) disabled.