Customer's passwords for mail, database and FTP user got reset to a random ones after restoring the backup.
Customer has set a password while taking the backup and while restoring the backup, he has unselected the option:
"Provide the password. If you have forgotten your password, clear this option. Note that in this case, some sensitive data will not be restored properly. For example, user passwords will be replaced with random ones, the information about already installed APS apps will be lost, and so on.".
Make sure to set password when restore password protected backup. Additionally please check Plesk documentation.