Unable to restore WordPress website from a backup file in Plesk: Failed to reset cache for the instance #1:

Symptoms

• When trying to restore a WordPress website from a backup file via the WP Toolkit, one of the following errors is displayed:

  PLESK_ERROR: Failed to restore the extension wp-toolkit: Failed to reset the cache for instance #1: PHP Warning: require(/var/www/vhosts/example.com/httpdocs/wp-includes/pomo/mo.php): failed to open stream: No such file or directory in /var/www/vhosts/example.com/httpdocs/wp-settings.php on line 116
  
  PLESK_ERROR: Failed to restore the extension wp-toolkit: Failed to reset cache for the instance #86: Error: WP-CLI needs WordPress 3.7 or later to work properly. The version currently installed is .

• The instance is displayed as broken at WordPress > example.com

Cause

A WordPress core file is missing or wp-includes/version.php file is empty.

Resolution

In order to resolve the issue, Re-install the WordPress core via the following steps:

1. Log into Plesk
2. Go to WordPress > example.com > Check WordPress Integrity
3. Click on the button Reinstall WordPress Core