Rails 3 returning a HTTP 406 Not Acceptable?

I have the following controller code:

  def create
    @admin = Admin.new(params[:admin])
    respond_to do |format|
      if @admin.save
        redirect_to(@admin, :notice => 'Admin was successfully created.')
      else
        render :action => "new"
      end
    end
  end

  def update
    @admin = Admin.find(params[:id])
    respond_to do |format|
      if @admin.update_attributes(params[:admin])
        redirect_to(admin_admins_path, :notice => 'Admin was successfully updated.')
      else
        render :action => "edit"
      end
    end
  end

and the following routes:

           admin_admins GET    /admin/admins(.:format)            {:action=>"index", :controller=>"admin/admins"}
           admin_admins POST   /admin/admins(.:format)            {:action=>"create", :controller=>"admin/admins"}
        new_admin_admin GET    /admin/admins/new(.:format)        {:action=>"new", :controller=>"admin/admins"}
       edit_admin_admin GET    /admin/admins/:id/edit(.:format)   {:action=>"edit", :controller=>"admin/admins"}
            admin_admin GET    /admin/admins/:id(.:format)        {:action=>"show", :controller=>"admin/admins"}
            admin_admin PUT    /admin/admins/:id(.:format)        {:action=>"update", :controller=>"admin/admins"}
            admin_admin DELETE /admin/admins/:id(.:format)        {:action=>"destroy", :controller=>"admin/admins"}

Now, aside from the slightly whacky naming - the redirects always result in a 406 Not acceptable. What could be wrong?

Remove respond_to do |format| blocks. Because you are not specifying to what format are you responding, e.g. format.html { #your code here } . Check documentation of respond_to how to use it properly.

I had a similar error, my controller was only responding to JSON. I needed it to respond to HTML also for the tests to work (which only makes sense):

class AdsController < ApplicationController
  respond_to :json, :html

I received the error when trying to do: assert_redirected_to ad_url(ad)

I started having this issue after a deploy in production, even tough everything was working fine in development.

After some 15 minutes of wasted time, I finally found out that I had forgotten to commit some of the view files (like index.html.erb).

Using tail -f log/production.log on the server revealed: FATAL -- : ActionController::UnknownFormat (SomeController#index is missing a template for this request format and variant.

In development the error didn't happen because, obviously, the view file was present.