Unable to register remote MySQL server in Plesk: Host is not allowed to connect to this MySQL server

Symptoms

• Upon the attempt to register a database server in Plesk > Tools & Settings > Database Servers, the following error is displayed:

  PLESK_ERROR: Error: The test connection to the database server has failed because of network problems:
  
  Host '203.0.113.2' is not allowed to connect to this MySQL server

  ---

  PLESK_ERROR: Error: The test connection to the database server has failed because of network problems:
  
  Unknown error 1130

• Only local users are registered in the database server:

  MYSQL_LIN: mysql> SELECT User, Host FROM mysql.user;
  +------------------+-----------+
  | User             | Host      |
  +------------------+-----------+
  | mysql.infoschema | localhost |
  | mysql.session    | localhost |
  | mysql.sys        | localhost |
  | root             | localhost |
  +------------------+-----------+
  4 rows in set (0.00 sec)

Cause

'root' user is not allowed to access the MySQL database server remotely.

Resolution

Log in to the MySQL on the remote database server and create a user who is allowed to access database server remotely from 203.0.113.2:

MYSQL_LIN: mysql> CREATE USER 'root'@'203.0.113.2' IDENTIFIED BY 'root_password';
mysql> GRANT ALL PRIVILEGES ON *.* TO 'root'@'203.0.113.2' WITH GRANT OPTION;
mysql> FLUSH PRIVILEGES;

Or a database user who can access the server from any host:

MYSQL_LIN: mysql> CREATE USER 'root'@'%' IDENTIFIED BY 'root_password';
mysql> GRANT ALL PRIVILEGES ON *.* TO 'root'@'%' WITH GRANT OPTION;
mysql> FLUSH PRIVILEGES;