[ghisirds]

Great, I've fixed it!!!

After a lot of trouble and reading again the vBulletin manual I've found an incoherence in config.php file.

It's in this section:

    //    ****** MASTER DATABASE USERNAME & PASSWORD ******
    //    This is the username and password you use to access MySQL.
    //    These must be obtained through your webhost.
$config['MasterServer']['username'] = 'root';
$config['MasterServer']['password'] = '';

The last two lines were duplicated, the first with a wrong username.

I can't understand how it can be happened but removing that 2 lines the error doesn't appear anymore.

Thanks to everybody.

Bye