Hi Christophe,
Please follow the KBA 1311166to find the duplicate SPNs in the netwrok.
If it does not work even after following Raunak's suggestion then follow the steps below.
- Delete the SPN: HTTP/boxi.domain.fr
- Create the SPN: HTTP/BOXI.DOMAIN.FR
- Restart the application server and test the SSO.
~SwapnilY