mysqlnd cannot  connect to MySQL 4.1+ using the old insecure authentication. Please use  an administration tool to reset your password with the command SET  PASSWORD = PASSWORD('your_existing_password'). This will store a  new, and more secure, hash value in mysql.user. If this user is used in  other scripts executed by PHP 5.2 or earlier you might need to remove  the old-passwords flag from your my.cnf file

Rješenje:
Updejtamo stari password istim takvim pomoću sljedeće naredbe:

UPDATE mysql.user SET Password=PASSWORD('lozinka') WHERE User='korisnik';

FLUSH PRIVILEGES;