Problem:
#2000 – 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
Cause:
In preparation for the rolling upgrade to PHP 5.3, we have changed all shared MySQL servers to use the a newer, more secure MySQL password hashing method. Unfortunately this may cause problems if you attempt to login using phpMyAdmin or Navicat Lite as they both use the password hashing specified by the server. Because your password may be stored as the old hashing method and the client application uses the new password hashing, you will receive this error.
Solution:
To fix this, you only need to reset the password in the control panel. This will cause the password to re-hash using the new 64 bit method and you will be able to login.
Content retrieved from: https://support.appliedi.net/kb/a1045/mysqlnd-cannot-connect-to-mysql-4_1-using-the-old-insecure-authentication_.aspx.