The solution!
1-
Edit C:\Program Files\SWsoft\Plesk\MySQL\Data\my.ini
Add at the end
[MySQLD]
skip-grant-tables
Restart Plesk SQL Server service
2-
Start, Run cmd
cd "\Program Files\SWsoft\Plesk\MySQL\bin"
mysql -P8306 mysql
mysql>update user set password=password('your_password') where user='horde';
mysql>exit
3-
Edit C:\Program Files\SWsoft\Plesk\MySQL\Data\my.ini
Remove
[MySQLD]
skip-grant-tables
Restart Plesk SQL Server service
4-
Edit inetpub\vhosts\webmail\horde\config\conf.php
Update:
$conf['sql']['port'] = 8306;
$conf['sql']['protocol'] = 'tcp';
$conf['sql']['hostspec'] = 'localhost';
$conf['sql']['username'] = 'horde';
$conf['sql']['password'] = 'your_password';
..and that's all folks!!