WordPress instance is marked as broken in Plesk: Uncaught TypeError: Argument 1 passed to WP_CLI

Symptoms

• WordPress instance is marked as broken in Plesk:

  PLESK_WARN: Broken instance on 'example.com' in '/httpdocs'

• Attempt to Refresh or Clone the instance leads to the next error message:

  PLESK_ERROR: PHP Fatal error: Uncaught TypeError: Argument 1 passed to WP_CLI\\Runner::WP_CLI\\{closure}() must be an instance of Exception, instance of ParseError given in /opt/psa/admin/plib/modules/wp-toolkit/vendor/wp-cli/wp-cli/php/WP_CLI/Runner.php:1470\nStack trace:\n
  #0 [internal function]: WP_CLI\\Runner-\x3EWP_CLI\\{closure}(Object(ParseError))\n
  #1 {main}\n thrown in /opt/psa/admin/plib/modules/wp-toolkit/vendor/wp-cli/wp-cli/php/WP_CLI/Runner.php on line 1470\n{"err_code":0,"err_message":"Failed to parse wp-config.php: Uncaught TypeError: Argument 1 passed to WP_CLI\\\\Runner::WP_CLI\\\\{closure}() must be an instance of Exception, instance of ParseError given in \\/opt\\/psa\\/admin\\/plib\\/modules\\/wp-toolkit\\/vendor\\/wp-cli\\/wp-cli\\/php\\/WP_CLI\\/Runner.php:1470\\nStack trace:\\n

• There is a Byte-Order Mark character in wp-config.php:
  
  

  # less /var/www/vhosts/example.com/httpdocs/wp-config.php
  <EF><BB><BF><?php
  /**
  * The base configuration for WordPress
  *
  * The wp-config.php creation script uses this file during the
  * installation. You don't have to use the web site, you can
  * copy this file to "wp-config.php" and fill in the values.
  [...]

• The same happens if BOM character is present in wp-includes/functions.php 

Cause

BOM (Byte-Order Mark) character doesn't allow WordPress toolkit to operate properly.

Resolution

1. Log into Plesk.
2. Go to Domains > example.com > File Manager and open wp-config.php or wp-includes/functions.php file.
3. Remove BOM character from the file and save the changes. BOM character should look like a dot (refer to the image above).
4. Refresh the instance via Domains > example.com > WordPress > Refresh.

It is also possible to remove all BOM characters automatically via the command line:

1. Create a backup of original file:

   # cp -a /var/www/vhosts/example.com/httpdocs/wp-config.php{,.bkp}

   Or

   cp -a /var/www/vhosts/example.com/httpdocs/wp-includes/functions.php{,.bkp}

2. Clear all BOM characters included in wp-config.php with following command:

   # sed -i '1 s/^\xef\xbb\xbf//' /var/www/vhosts/example.com/httpdocs/wp-config.php

   Or

   # sed -i '1 s/^\xef\xbb\xbf//' /var/www/vhosts/example.com/httpdocs/wp-includes/functions.php