Plesk is not accessible with error 500: Failed opening required

Symptoms

• Plesk is not accessible with error 500.

• The following error is found in C:\Program Files (x86)\Plesk\admin\logs\php_error.log:

  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

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

• Running Plesk commands fail with the following error:

  C:\Users\Administrator> plesk login -support
  'C:\Program Files (x86)\Plesk\admin\plib\cu\admin.php' is not recognized as an internal or external command, operable program or batch file.
  at (RunTime::Execute::commandLine line 595)
  at Unable to execute CU(wmain line 64)exit status 1

  Note: Other commands like plesk bin <script> leads to the same error

Cause

Corrupted Plesk installation / Antivirus software interfering with Plesk functionality.

Resolution

1. Connect to the server via RDP

2. Open Plesk Service Monitor and stop Plesk Management Service

3. Create a copy of your admin folder in C:\Program Files (x86)\Plesk into a different location like C:\

4. Download plesk-core.zip from https://autoinstall-win.plesk.com/, extract and substitute files under %plesk_dir%admin\plib.

   Note: As an example, if Plesk is version 18.0.72, the plesk-core.zip file is located in https://autoinstall-win.plesk.com/PANEL-WIN_18.0.72/dist-windows/

5. Enable back Plesk Management Service from Plesk Service Monitor

To avoid this to happen again, add %plesk_dir% to exceptions on antivirus.