Some web sites are down, the following error is displayed:
Database Connection Failed Error: Client does not support authentication protocol requested by server; consider upgrading MySQL client (1251) Function Performed: CONNECT Host/DB: localhost/testdb Database Type: mysql</font></span>
The root cause of the issue lies within the new password hashing method used by MySQL 4.1.1 and higher:
In older versions of PHP, the MySQL extension does not support the authentication protocol used in MySQL 4.1.1 and higher. This is true regardless of the PHP version being used. If you wish to use the mysql extension with MySQL 4.1 or newer, you may need to follow one of the options discussed in this thread for configuring MySQL to work with old clients. The mysqli extension (stands for "MySQL, Improved"; added in PHP 5) is compatible with the improved password hashing employed in MySQL 4.1 and higher, and no special configuration of MySQL needs be done to use this MySQL client library. More information can be found in the same thread.
The common solution in this situation is to upgrade PHP version to the one that supports new password hashing method. Since newer Parallels Plesk Panel versions allow choosing the PHP version on per domain basis, in some cases it is enough to switch PHP version for the domain to a newer one (go to Websites and domains -> Hosting Settings for domain.tld -> General). However if you do not have the newer PHP version installed on the server at all, you will need to install it first.