Plesk is not accessible with error 500: Failed opening required

Symptoms

• Plesk is not accessible with error 500.

• The following error is found in php_error.log:

  CONFIG_TEXT: PHP Fatal error: require_once(): Failed opening required 'C:\Program Files (x86)\Plesk\admin\plib/vendor/autoload.php' (include_path='C:\Program Files (x86)\Plesk\admin\plib') in C:\Program Files (x86)\Plesk\admin\plib\Loader.php on line 20

  CONFIG_TEXT: 'C:\Program Files (x86)\Plesk\admin\plib\cu\admin.php' is not recognized as an internal or external command, operable program or batch file.

Cause

Corrupted Plesk installation / Antivirus software interfering with Plesk functionality.

Resolution

Add %plesk_dir% to exceptions on antivirus.

In case files mentinoned in the error message are missing, download plesk-core.zip from https://autoinstall-win.plesk.com/, extract and substitute files under %plesk_dir%admin\plib.