[kh99]

OK, I tried this code and it worked for me (it inserted a user and there was no db error):

<?php

// vBulletin
define('REGISTERED_USERGROUP', 2); // typical default for registered users
define('PERMANENT_COOKIE', false); // false=session cookies (recommended)
define('THIS_SCRIPT', __FILE__); 
define('VB_AREA', 'External');
$dir = getcwd();
chdir('forum4.2.0/');
require_once('./includes/init.php'); // includes class_core.php
require_once('./includes/class_dm.php'); // for class_dm_user.php
require_once('./includes/class_dm_user.php'); // for user functions
require_once('./includes/functions.php'); // vbsetcookie etc.
require_once('./includes/functions_login.php'); // process login/logout 
chdir($dir); 

class Forum2 {
    
    private $dmuser;
    
    public function __construct()
    {
        global $vbulletin;
        $this->dmuser =& datamanager_init('User', $vbulletin, ERRTYPE_ARRAY);
    }
    
    public function register($data = array(), $login = FALSE)
    {
        global $vbulletin;
        foreach ($data as $key => $value) {
            if (!is_array($value)) {
                $this->dmuser->set($key, $value);
            }
        }
        if (isset($data['options']) && is_array($data['options'])) {
            foreach ($data['options'] as $key => $value) {
                $this->dmuser->set_bitfield('options', $key, $value);
            }
        }
        $this->dmuser->pre_save();
        //print_r($this->dm);
        if (count($this->dmuser->errors) > 0) {
            return implode('<li>', $this->dmuser->errors);
        }
        return $this->dmuser->save();
    }
}

$f2 = new Forum2();

        $f2->register(array(
            'username'    => 'testing5',
            'email'        => 'test@test5.com',
            'password'    => 'password5'
        ));

other than removing your controller class and changing the directory, the only thing I changed was the part in red, which said "dm" instead of "dmuser".

ETA: I was thinking you might throw this is a php file and try it, to see if the problem is somewhere else.