Help tracking down error - Failed opening required

[papaschloss]

I have some code in a directory in my forum directory that keeps causing the following error whenever I call it:

Failed opening required 'DIR/includes/class_dm.php' (include_path='.:/usr/share/php:/usr/share/pear')

I've traced the problem to this point:
1) My function calls process_logout() in functions_login.php. (I have included functions_login.php with require_once - that is working and not an issue here).
2) In process_logout() on line 341 (I'm running version 3.6.8) the following call is made:

$userdata =& datamanager_init('User', $vbulletin, ERRTYPE_SILENT);

This is where the problem starts.
3) The function datamanager_init() is in the file functions.php. In this file, at line 74, the following call is made:

require_once(DIR . '/includes/class_dm.php');

4) DIR is defined in init.php with the following line:

if (!empty($vbulletin->config['Misc']['forumpath']))
{
     define('DIR', $vbulletin->config['Misc']['forumpath']);
}

5) I checked my config.php and I do have this defined properly:

$config['Misc']['forumpath'] = '/var/www/testvb';

So that's as far as I've gotten. If anyone can see something that I can't here then please tell me. I'm at an impasse right now.

Thanks!