The IBAN in the payment account (BT-84) fails the check-digit validation (ISO 13616, mod 97) — usually a typo or transposed digits.
Compare the IBAN with the bank statement and correct it. Spaces are allowed and not an error.
The Billhorse validator checks XRechnung, ZUGFeRD and Factur-X right in your browser — your file is never uploaded.
Open the validator ← All validation rules
Rule source: EN 16931 / XRechnung (KOSIT) or Billhorse check (BH-*).