UaMV I suppose you have a rule in your webserver configuration somewhere that prevents access to that url. I don't see where the Access Denied could come from.
If the extension wasn't working, you'd get a "Method Not Allowed" error (with a base Flarum install)
If you call
m.route('/login') in the console from the homepage, it works, so this means the extension correctly registered the url, at least on the client side.
Or maybe there's another extension registering that route as well. If you don't find anything in your webserver preventing the access, could you open an issue on our bug tracker with the output of
php flarum info ? You could also try disabling all other extensions to see if that fixes it.