Plesk is not accessible after upgrade: 500 Internal Error

Symptoms

• Plesk is not accessible with one of the errors below:

  PLESK_INFO: 500 Internal Error

  PLESK_INFO: Fatal error: Class 'CommonPanel_Config_Defaults' not found in C:\Program Files (x86)\Parallels\Plesk\admin\plib\Plesk\Config.php on line 14
  Fatal error: Class 'CommonPanel_Config_Defaults' not found in C:\Program Files (x86)\Parallels\Plesk\admin\plib\Plesk\Config.php on line 14

  CONFIG_TEXT: ERROR: Cannot redeclare Clue\StreamFilter\append() (previously declared in C:\Program Files (x86)\Plesk\admin\plib\vendor\clue\stream-filter\src\functions.php:17) (functions.php:27)

• Plesk may show just an empty page.

• The following errors on attempt to execute plesk login command:

  C:\> plesk login
  exit status 255

• Errors in Event Viewer:

  CONFIG_TEXT: Execute plesksrvstartuphandler.exe C:\Program Files "(x86)\Plesk\admin\bin\plesksrv.exe" failed with error code 255:
  at (execPleskUtil line 106)
  at Execute C:\Program Files (x86)\Plesk\admin\bin\plesksrvstartuphandler.exe C:\Program Files "(x86)\Plesk\admin\bin\plesksrv.exe"(execPleskUtil line 111)
  at Call startup handler(plesksrv::StartupHandlerThread::runProc line 495)
  Execute file name: C:\Program Files (x86)\Plesk\admin\bin\plesksrv.exe

• One of the errors below is observed in  %plesk_dir%\admin\log\php_error.log

  PLESK_INFO: PHP Parse error: syntax error, unexpected 'KŒûØ·' (T_STRING) in C:\Program Files (x86)\Plesk\admin\plib\api-common\cu.php on line 70
  PHP Parse error: syntax error, unexpected '!' in C:\Program Files (x86)\Plesk\admin\plib\api-cli\CliUtilityRunner.php on line 36

  CONFIG_TEXT: PHP Fatal error: require(): Failed opening required 'C:\Program Files (x86)\Plesk\admin\plib\vendor\composer/../react/promise/src/functions_include.php' (include_path='C:\Program Files (x86)\Plesk\admin\plib\vendor/plesk/zf1/library;C:\Program Files (x86)\Plesk\admin\plib') in C:\Program Files (x86)\Plesk\admin\plib\vendor\composer\autoload_real.php on line 70

  CONFIG_TEXT: ERROR: Cannot redeclare Clue\StreamFilter\append() (previously declared in C:\Program Files (x86)\Plesk\admin\plib\vendor\clue\stream-filter\src\functions.php:17) (functions.php:27)

Cause

Corrupted PHP library files in subfolders of C:\Program Files (x86)\Plesk\admin\ folder.

Resolution

1. Connect to the server via RDP

2. Remove and replace the following folders from test server:

   CONFIG_TEXT: C:\Program Files (x86)\Plesk\admin\htdocs
   C:\Program Files (x86)\Plesk\admin\bin
   C:\Program Files (x86)\Plesk\admin\plib

3. Restart Plesk services How to restart the Plesk service

4. Restart IIS using the following command in a command prompt:

   C:\> iisreset /restart