xxx'@'localhost' (using password: YES)

[TheLastSuperman]

Check database in cpanel/plesk and compare to database listed in /includes/config.php - do they match? Is there a user assigned to the database from "MySQL Databases" in cpanel?

You need to edit config.php and ensure your information provided in the file, is correct. "xxx" is probably not the account name in the path unless you're a o.O type of site! xxx = accountname below.

These lines in config.php need to be changed:

$config['Database']['dbname'] = 'accountname_databasename';
$config['MasterServer']['username'] = 'accountname_databaseusername';
$config['MasterServer']['password'] = 'databaseusernamepassword';

Yours will have something along the lines of:

$config['Database']['dbname'] = 'xxx_databasename';
$config['MasterServer']['username'] = 'xxx_databaseusername';
$config['MasterServer']['password'] = 'databaseusernamepassword';