Cannot access MailEnable webmail on Plesk for Windows server: 503 Service Unavailable

Symptoms

• On a Plesk for Windows server, MailEnable webmail shows 503 Service Unavailable.

• The IIS application pool MailEnableAppPool crashes on the attempt to access webmail.example.com.

• Event Viewer shows events with ID 2280 and 5139 five times in a row. After that, the Event with ID 5002 appears and the application pool is stopped:

  • Event ID 2280:

    PLESK_INFO: The Module DLL C:WindowsSystem32inetsrvModSecurityIIS.dll failed to load. The data is the error.

  • Event ID 5139:

    PLESK_INFO: A listener channel for protocol ‘http’ in worker process ‘12864’ serving application pool ‘MailEnableAppPool’ reported a listener channel failure. The data field contains the error number.

  • Event ID 5002:

    PLESK_INFO: Application pool ‘MailEnableAppPool’ is being automatically disabled due to a series of failures in the process(es) serving that application pool.

• Switching the option Enable 32-Bit Applications to True value in Start > Server Manager > Tools > Internet Information Services (IIS) Manager > Application Pools > MailEnableAppPool > Advanced Settings works around the issue:
  

Cause

The module ModSecurityIIS.dll cannot be loaded due to missing several Microsoft Visual C++ Redistributables with x64 architecture.

Resolution

1. Connect to the server using RDP.

2. Install all the packages vcredistXX_x64.msi and vcredistXXX_x64.msi from the folder %SystemDrive%ParallelsInstallerparallelsPANEL-WIN_17.5.3thirdparty-msi-Windows-any-x86_64.

   Note: Replace 17.5.3 with the version of Plesk installed on the server.

3. Open the IIS Manager in Start > Server Manager > Tools > Internet Information Services (IIS) Manager and restart the application pool MailEnableAppPool  in server_hostname > Application Pools > MailEnableAppPool > Stop/Start: