mysqladmin -u root -p'oldpass' password 'roottest'

mysql -u root -p'roottest'
mysql> use dbispconfig;
mysql> database changed
mysql> update sys_user set passwort = md5('admintest') WHERE username = 'admin';
Query OK, 1 row affected (0.07 sec)
Rows matched: 1  Changed: 1  Warnings: 0

vi /usr/local/ispconfig/server/lib/mysql_clientdb.conf
<?php
$clientdb_host                  = 'localhost';
$clientdb_user                  = 'root';
$clientdb_password              = 'roottest';
?>

vi /usr/local/ispconfig/server/lib/config.inc.php

...
$conf['db_password'] = 'VERYLONGPASSWORDSTRING';
...

mysql -u root -p
mysql> use mysql
mysql> update User set Password = PASSWORD('VERYLONGPASSWORDSTRING') WHERE User = 'ispconfig';