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

Symptoms

• 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)

• Upon the attempt to register database server in 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

  Or:

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

Cause

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

Resolution

Log in to the MySQL on the remote database server and create a user which 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 database user which 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;