Symptoms

when logged in to the Parallels Plesk Panel Control Panel the following message appears:

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Warning: The site_isolation_config.ini file is broken or absent, the default configuration file will be used.
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Cause

The file which contains restrictions for the potentially Insecure Hosting Options usage is missing in the system or corrupted.

Resolution

Backup the broken file /usr/local/psa/admin/conf/site_isolation_settings.ini in case it contains the adjusted settings
cp /usr/local/psa/admin/conf/site_isolation_settings.ini{,.orig}
  Replace the corrupted file with the the default configuration one:
cp /usr/local/psa/admin/conf/site_isolation_settings.ini.default /usr/local/psa/admin/conf/site_isolation_settings.ini
To rebuild the configuration or adjust settings from scratch see the Parallels Plesk Panel administrator's guide for details:
            Restricting the Use of Potentially Insecure Hosting Options