ERROR PleskMainDBException Access denied for user 'admin'@'localhost' (using password: YES)

Add the skip-grant-tables option under the [PleskSQLServer] section of the file %plesk_dir%MySQL\my.ini.

Then, restart the Plesk SQL Server service:

C:\> net stop PleskSQLServer && net start PleskSQLServer

Then change the admin password:

C:\> plesk db "update mysql.user set authentication_string=password('new_password') where user='admin'"

Remove the skip-grant-tables and restart the service again.

Connect to psa database and list database servers:

C:\>"%plesk_dir%\mysql\bin\mysql.exe" -uadmin -P8306 -p psa
Enter password: ***********
mysql> select * from databaseservers;
+----+-------------------+------+-------+----------------+-------------+-------------------------------------------------------+
| id | host | port | type | server_version | admin_login | admin_password |
+----+-------------------+------+-------+----------------+-------------+----------------+-------------------------------------------+
| 1 | localhost | 3306 | mysql | 5.1.68 | admin | $AES-128-CBC$dOWumr1ccMryZe7HTKh+dA$tMQ0kteqadixYOK51a2Nmg |
| 2 | .\MSSQLSERVER2008 | 0 | mssql | 10.50.1600 | sa | $AES-128-CBC$Mp/U5Dyj3Dp+u40mRnEkKA$M/EC+USxJjKfXUoXheqW2g |
+----+------------------------+------+-------+----------------+-------------+-------------------------------------------------------+

Temporarily remove the row which describes the MySQL instance:

MYSQL_WIN: mysql> delete from databaseservers where id=1;
Query OK, 1 row affected (0.00 sec)

Go to Plesk > Tools and Settings > Database servers and add a mysql database server with the admin username and your own password.

Check the newly added row in the databaseservers table and take the value from the admin_password column.

MYSQL_WIN: select * from databaseservers;
+----+-------------------+------+-------+----------------+-------------+------------------------------------------------------------+---------------+------------+
| id | host | port | type | server_version | admin_login | admin_password | parameters_id | last_error |
+----+-------------------+------+-------+----------------+-------------+------------------------------------------------------------+---------------+------------+
| 2 | .\MSSQLSERVER2008 | 0 | mssql | 10.50.1600 | sa | $AES-128-CBC$Mp/U5Dyj3Dp+u40mRnEkKA$M/EC+USxJjKfXUoXheqW2g | 0 | no_error |
| 5 | localhost | 3306 | mysql | 5.1.68 | admin | $AES-128-CBC$G2NUq/EH0zPCHYxJeRiY8g$Up7gweyCwXl3I613+GTcDg | 189 | no_error |
+----+-------------------+------+-------+----------------+-------------+------------------------------------------------------------+---------------+------------+

Revert changes in this table back to their original state and set the new password only for example:

MYSQL_WIN: mysql> delete from databaseservers where id=5;
Query OK, 1 row affected (0.00 sec)
mysql> INSERT INTO `databaseservers` VALUES (1,'localhost','3306','mysql','5.1.68','admin','$AES-128-CBC$dOWumr1ccMryZe7HTKh+dA$tMQ0kteqadixYOK51a2Nmg',NULL,'no_error');
Query OK, 1 row affected (0.00 sec)
mysql> UPDATE databaseservers SET admin_password='$AES-128-CBC$G2NUq/EH0zPCHYxJeRiY8g$Up7gweyCwXl3I613+GTcDg' where id=1;
Query OK, 1 row affected (0.00 sec)Rows matched: 1 Changed: 1 Warnings: 0