If you change MySQl root password on the linux server with zPanel control panel, you may get error like:
in your zpanelCritical Error:  - Unable to connect or authenticate to the ZPanel database (zpanel_core).
We advice that you contact the server administator to ensure that the database server is online and that the correct connection parameter are being used.
this is because you need to tell zpanel new mysql root password. Change the password in these files:
(just search old password and replace by new one)
How to change mysql root password?
What is default zpanel mysql root password?mysqladmin -u root -pYourOldMysqlPassword password YourNewMysqlPassword
i think it was generated randomly, and its stored in /root/password/passwords.txt