When opening Plesk Onyx 17.5.3 or Plesk Onyx 17.8.11, the following error message is shown in a web-browser:
The file /opt/psa/admin/htdocs/application.php is part of Plesk distribution. It cannot be run outside of Plesk environment.
The issue is with Domain Traffic Monitor extension on Plesk Onyx 17.5.3 / Plesk Onyx 17.8.11 with old update.
Connect to the Plesk server via SSH.
Temporarily disable the Domain Traffic Monitor extension:
# plesk bin extension --disable traffic-monitor
Apply one of the following steps:
Enable the extension back.