url_for() helpers are namespaced when adding plug-in engines. add forward /
from Rails documetation: http://apidock.com/rails/ActionController/Base/url_for
If the controller name begins with a slash no defaults are used:
url_for :controller => '/home'
In particular, a leading slash ensures no namespace is assumed. Thus, while url_for :controller => 'users' may resolve to Admin::UsersController if the current controller lives under that module, url_for :controller => '/users' ensures you link to ::UsersController no matter what.