Hmm, it's trying to spit out an error message, but it can't actually find the language file. Not exactly sure why, but a quick patch should let you at least get around it. You can edit the libs/ubbthreads.inc.php file. Line 892 looks like this:

include("languages/{$user['USER_LANGUAGE']}/generic.php");

Change that, to this:

include("{$config['FULL_PATH']}/languages/{$user['USER_LANGUAGE']}/generic.php");