From what I understand about your setup, you have two URLs pointing at your forum, with the first one redirecting to the second one. I’m not sure if an option should be added to the plugin for dealing with that situation.
For now, instead of editing the plugin’s code, you can add to the allowed hosts array in your theme’s functions.php file by doing something like this:
function your_namespace_allow_extra_redirect( $hosts ) {
$hosts[] = 'yourseconddomain.com';
return $hosts;
}
add_filter( 'allowed_redirect_hosts', 'your_namespace_allow_extra_redirect' );