Problem with 1.2->1.4 upgrade

Hi,

I just tried to upgrade my 1.2 version to 1.4 and followed the instructions.
Everything was ok (copying files, DB update, config.php update...) using db_update.php, but when the whole process finished, I returned to the index page and got an this error message :

Sorry! The page could not be loaded.
Your PunBB database is out-of-date and must be upgraded in order to continue.
Please run db_update.php in order to complete the upgrade process.

And when I try to run db_update.php again, i obtain this message :

Sorry! The page could not be loaded.
Your database is already as up-to-date as this script can make it.

So... can someone explain me what to do in this weird case ?

Thanks