It seems to be a bug in the incorrect use of require_once in anti-spam/includes/scanner/classes/scanner/boot.php
When PEAR FILE package is installed (https://pear.php.net/package/File) require_once includes the File.php from Pear instead of anti-spam/includes/scanner/classes/scanner/File.php file.
Fix should be something like this (similar is used in anti-spam/libs/factory/core/boot.php):
<?php
// Exit if accessed directly
//if ( ! defined( 'ABSPATH' ) ) {
// exit;
//}
define('SCANNER_DIR', dirname(__FILE__));
require_once SCANNER_DIR . '/File.php';
require_once SCANNER_DIR . '/Result.php';
require_once SCANNER_DIR . '/Scanner.php';
require_once SCANNER_DIR . '/Signature.php';
require_once SCANNER_DIR . '/SignaturePool.php';
require_once SCANNER_DIR . '/HashListPool.php';
