For a temp fix on this you should be able to edit libs/ubbthreads.inc.php. Line 1436 of that file there is this block of code:

Code
        if (!preg_match("/^\w+$/",$name)) {
            return array(false, 'BAD_LNAME');
        }

You can change that, to this and it will allow for your spaces:

Code
        if (!preg_match("/^\w+$/",$name)) {
            //return array(false, 'BAD_LNAME');
        }