Unable to access Plesk: HTTP Error 500: PHP Fatal error: Unknown: Failed opening required 'auth.php' (include_path='.:') in Unknown on line 0

Symptoms

• Plesk Obsidian running on a Linux-based operating system

• When trying to run any Plesk-related command (plesk repair,  plesk login etc.) via a terminal, the following output is shown:

  CONFIG_TEXT: PHP Warning: require_once(api-cli/CliUtilityRunner.php): failed to open stream: No such file or directory in /usr/local/psa/admin/plib/api-cli/admin.php on line 4
  PHP Fatal error: require_once(): Failed opening required 'api-cli/CliUtilityRunner.php' (include_path='.:') in /usr/local/psa/admin/plib/api-cli/admin.php on line 4
  exit status 255

• Plesk GUI is not accessible with the error 500

• The following errors can be found in /var/log/plesk/panel.log file:

  CONFIG_TEXT: PHP Warning: Unknown: failed to open stream: No such file or directory in Unknown on line 0
  [01-Nov-2019 19:26:31 UTC] PHP Fatal error: Unknown: Failed opening required 'auth.php' (include_path='.:') in Unknown on line 0

Cause

Invalid syntax is present in the /usr/local/psa/admin/conf/php.ini file of the server.

Resolution

Copy the file /usr/local/psa/admin/conf/php.ini from a fresh server with the same OS and Plesk version and replace the one on the affected server with it.