ActionController::UnknownFormat

[pencilcheck]

http://stackoverflow.com/questions/19193222/rails-4-devise-3-1-1-actioncontrollerunknownformat-in-deviseregistrationscon

I encounter something similar to the issue above on SO when setting up devise for rails admin with devise 3.4.1.

The problem lies in that in devise/sessions/new.html.erb the resource_name is passed along to generate the url for the form, if I took out all the references to resource_name like so:

<%= form_for(resource, url: session_path) do |f| %>

Then the app will start working.

[josevalim]

It seems to be a conflict with Rails admin. The session_path that Devise uses is a Devise specific helper that will invoke user_session_path, based on the resource name. If it is generating something else, it is because another part of the application defines a session_path that conflicts with Devise one.