The cause:
When this error comes up, all of the Pleo expenses in the queue have failed to go through the Datev. This happens when using a module in Datev called “Belegfreigabe online”. Unfortunately, this module does not work with Rechnungsdatenservice 1.0, which is the mandatory module for the Pleo integration. This is a limitation on Datev's side.
The solution:
Due to how Datev works, there is no fix as such. The workarounds are to either
Disable Belegfreigabe online (if not needed)
Switch from using the DUO integration to using a file export such as Datev CSV