00100: Signing certificate expired
What it means
SdI code 00100 means the certificate used to sign the file had expired.
The usual cause
Check the validity period of the certificate used to sign the file.
How to fix it
Renew the signing certificate with your certification authority or use a valid one, sign the file again and send it under a new name.
The code as published
Read the official document . Source: Elenco dei controlli effettuati sul file fattura del Sistema di Interscambio, version 2.0, published by Agenzia delle Entrate, fatturapa.gov.it, read on 8 November 2026.
Klarbon explains each code in its own words; the official wording is in the document linked above.