Plesk shows HTTP Error 503. The service is unavailable: The identity of application pool PleskControlPanel is invalid

Symptoms

• Plesk is not accessible with the following error:

  HTTP Error 503. The service is unavailable.

• In Event Viewer > Windows Logs > System, the following warnings are found:

  The identity of application pool PleskControlPanel is invalid. The user name or password that is specified for the identity may be incorrect, or the user may not have batch logon rights. If the identity is not corrected, the application pool will be disabled when the application pool receives its first request. If batch logon rights are causing the problem, the identity in the IIS configuration store must be changed after rights have been granted before Windows Process Activation Service (WAS) can retry the logon. If the identity remains invalid after the first request for the application pool is processed, the application pool will be disabled. The data field contains the error number.

  Application pool PleskControlPanel has been disabled. Windows Process Activation Service (WAS) did not create a worker process to serve the application pool because the application pool identity is invalid.

  Application pool PleskControlPanel has been disabled. Windows Process Activation Service (WAS)

• Plesk installer may show this error when performing an upgrade:

  Plesk Installer has stopped working
  Problem signature: 
  Problem Event Name: APPCRASH 
  Application Name: parallels_installer_Microsoft_6.3_x86_64_3.22.12.exe 
  Application Version: 3.22.12.0 
  Application Timestamp: 5aa63df6 
  Fault Module Name: parallels_installer_Microsoft_6.3_x86_64_3.22.12.exe 
  Fault Module Version: 3.22.12.0 
  ...

Cause

Incorrect user credentials specified for Plesk application pool PleskControlPanel, or psaadm user was modified.

Resolution

1. Connect to the server via RDP.
2. Open Start > Plesk Reconfigurator as administrator.
3. Click Repair Plesk Installation.
4. Select Plesk Server Accounts.
5. Click Check.