Hi Bertrand:
Can you provide some additional information on your configuration and the steps you have taken to this point? The (-450) error is stating that the ArcSDE binaries are not compatible with the geodatabase release.
The error you are displaying states that the database has not been upgraded and thus the system tables it is looking for (ie. GDB_ITEMS, etc.) are not found.
Verify that this geodatabase has in fact been upgraded from 9.3.1 to 10.0 as you stated. Connect to the geodatabase from ArcCatalog using a direct-connection and right-click the connection. Go to "Properties and check the "Upgrade Geodatabase" button for information on the status of the ArcGIS client and ArcSDE geodatabase compatibility.
Also, verify that the ArcSDE release that you have installed is consistent with the database version that you have upgraded to. This should be consistent with the ArcGIS client that was used to perform the upgrade.
- Jon