The file /usr/local/ispconfig/server/lib/config.inc.php may not contain the mysql root user or password. If it conatisn the user root on your setup, then you have a installtion error. The mysql user used by ispconfig to connect with the database is always named "ispconfig". The file mysql_cientdb.conf contains the root user which is only used to create new mysql databases.
The error can also be caused by wrong settings in the mysql my.cnf file e.g. when you set timeout values too low.