The phpBB auth plugin for mdn.staging/forums is not authenticating people right, probably because the Dekiwiki password hash isn't exactly documented. I tried another variant in bug 579899. Please update /includes/auth/auth_mdc.php from SVN. Thanks!
Updated from SVN. I added this to cron, it updates every 15 minutes.
I can log in to staging forums with my Deki creds, so this looks fixed.