Unable to access Plesk Premium Email on Plesk Obsidian: Internal Server Error

Symptoms

• In an attempt to access the Plesk Premium Email on the fresh server with the Plesk Obsidian installed the following error is shown:  
  

  CONFIG_TEXT: 500 Internal Server Error
  The server encountered an internal error or misconfiguration and was unable to complete your request.

• In the Apache log file, the following errors can be found:
  
  • For Debian based systems (Ubuntu/Debian):

    In /var/log/apache2/error.log file:

    CONFIG_TEXT: [fcgid:warn] [pid 4703] (104)Connection reset by peer: [client 203.0.113.2:55628] mod_fcgid: error reading data from FastCGI server
    [core:error] [pid 4703] [client 203.0.113.2:55628] End of script output before headers: index.php

  • For RHEL based systems (CentOS/CloudLinux):
    In /var/log/httpd/error_log file:

    CONFIG_TEXT: [fcgid:warn] [pid 3894:tid 140570113193728] (104)Connection reset by peer: [client 192.0.2.2 :54950] mod_fcgid: error reading data from FastCGI server, referer: https://203.0.113.2/smb/email-address/list/domainId/1
    [Thu Nov 21 13:25:20.048694 2019] [core:error] [pid 3894:tid 140570113193728] [client End of script output before headers: index.php, referer: https://203.0.113.2/smb/email-address/list/domainId/1

Cause

The Plesk Premium Email relies on PHP by OS vendor.
However, the PHP by OS vendor is not installed by default in Plesk Obsidian.
The bug was fixed in the 16.11.0 version of the Plesk Premium Email extension.

Resolution

Update Plesk Premium Email extension to the latest version:

1. Log into Plesk.
2. Go to Extensions > Updates.
3. Press the Check Now button.
4. Select the Plesk Premium Email, powered by Kolab and press the Update button.

Note: if an update is not possible, as a workaround install PHP from OS vendor component, using one of the solutions below: