DE-R-026
A Peppol Germany national rule (DE-R-*) – applies when the invoice is sent over the Peppol network to a receiver in Germany.
What this error means
DE-R-026
When both seller and buyer are located in Germany and the invoice type is 384 (corrected invoice), a preceding invoice reference (BG-3) must be provided at least once.
How to fix it
Add cac:BillingReference/cac:InvoiceDocumentReference with the corrected invoice number.