Unable to upgrade Plesk: update-rc.d: error: insserv rejected the script header

Symptoms

• The following error occurs on attempt to upgrade Plesk:

  Setting up apache2 (2.4.18-2ubuntu3.5) ...
  insserv: script apache2: service lsws already provided!
  info: Executing deferred 'a2enmod php7.0' for package libapache2-mod-php7.0
  insserv: script apache2: service lsws already provided!
  insserv: exiting now!
  update-rc.d: error: insserv rejected the script header
  dpkg: error processing package apache2 (--configure):
  subprocess installed post-installation script returned error exit status 1
  E: Sub-process /usr/bin/dpkg returned an error code (1)
  Unable to upgrade Plesk: update-rc.d: error: insserv rejected the script header

• Apache2 is shown as half configured:

  # dpkg -l | grep apache2
  iF apache2 2.4.18-2ubuntu3.5 amd64 Apache HTTP Server

• Init script for Apache2 is a symbolic lint to the LiteSpeed init script:

  # ls -l /etc/init.d/apache2
  lrwxrwxrwx 1 root root 6 Mar 18 12:49 /etc/init.d/apache2 -> ./lsws

Cause

Incorrect script provided by LiteSpeed.

Resolution

1. Log into the server via SSH.
2. Remove a symbolic link:

   # unlink /etc/init.d/apache2

3. Restore the original Apache2 init script from the backup created by LiteSpeed:

   # cp -a /etc/init.d/apache2.ls_bak /etc/init.d/apache

4. Initialize the upgrade once again.

Another solution is to switch back to Apache via Extensions > LiteSpeed Extension.

Note: as LiteSpeed is not officially supported by Plesk, contact LiteSpeed support regarding the issue with /etc/init.d/lsws script in case it is required to use it as a symbolic link:
https://www.litespeedtech.com/support