I disagree with you that it cannot be a config.php setting. I disagree even more that you cannot check or update the config.php file. That wouldn't be much use if you couldn't.
Essentially, if this happens the username, password and hostname (localhost) don't agree with the GRANT you did after creating the database. I don't know anything about Plesk but I would review the username and password settings in the database setup against the contents of config.php (once you have worked out how to access it).
Essentially, if this happens the username, password and hostname (localhost) don't agree with the GRANT you did after creating the database. I don't know anything about Plesk but I would review the username and password settings in the database setup against the contents of config.php (once you have worked out how to access it).