Plesk is not accessible after upgrade: 500 Internal Error

Symptoms

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

  500 external Error

  Fatal error: Class 'CommonPanel_Config_Defaults' not found in C:Program Files (x86)ParallelsPleskadminplibPleskConfig.php on line 14
  Fatal error: Class 'CommonPanel_Config_Defaults' not found in C:Program Files (x86)ParallelsPleskadminplibPleskConfig.php on line 14

  ERROR: Cannot redeclare ClueStreamFilterappend() (previously declared in C:Program Files (x86)Pleskadminplibvendorcluestream-filtersrcfunctions.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:

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

• One of the errors below is observed in  %plesk_dir%adminlogphp_error.log

  PHP Parse error: syntax error, unexpected 'KŒûØ·' (T_STRING) in C:Program Files (x86)Pleskadminplibapi-commoncu.php on line 70
  PHP Parse error: syntax error, unexpected '!' in C:Program Files (x86)Pleskadminplibapi-cliCliUtilityRunner.php on line 36

  PHP Fatal error: require(): Failed opening required 'C:Program Files (x86)Pleskadminplibvendorcomposer/../react/promise/src/functions_include.php' (include_path='C:Program Files (x86)Pleskadminplibvendor/plesk/zf1/library;C:Program Files (x86)Pleskadminplib') in C:Program Files (x86)Pleskadminplibvendorcomposerautoload_real.php on line 70

  ERROR: Cannot redeclare ClueStreamFilterappend() (previously declared in C:Program Files (x86)Pleskadminplibvendorcluestream-filtersrcfunctions.php:17) (functions.php:27)

Cause

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

Resolution

1. Connect to the server via RDP

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

   C:Program Files (x86)Pleskadminhtdocs
   C:Program Files (x86)Pleskadminbin
   C:Program Files (x86)Pleskadminplib

3. Restart Plesk services How to restart the Plesk service

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

   C:> iisreset /restart