I've tested it once again with the newer versions of miniBB and WordPress, and I could confirm that this solution is still working.
IF you are experiencing troubles with login on forums, specially if you are testing this environment on Localhost, it may be because WordPress sets its primary authorization cookie for a specified folder only, not the whole domain or site as it's required by the miniBB common integration guide. To fix this, try to modify /wp-includes/default-constants.php
fixing the following:
if ( !defined('COOKIEPATH') )
//define('COOKIEPATH', preg_replace('|https?://[^/]+|i', '', get_option('home') . '/' ) );
define('COOKIEPATH', '/' );
So, first you should comment or remove the original setting build upon 'home' option, and re-specify it as a simple global path, which would work for all scripts placed under that folder.
If it doesn't work, you may try the same with 'SITECOOKIEPATH' and 'ADMIN_COOKIE_PATH' options.
Check your cookie paths using Mozilla's Tools -> Options -> Privacy - Show Cookies