Get 401 Unauthorized after login with session/new partial

[Tobias Schoenau]

Started POST "/login" for 127.0.0.1 at 2012-05-27 22:22:23 +0200
Processing by Devise::SessionsController#create as HTML
Parameters: {"utf8"=>"✓",
"authenticity_token"=>"dHDnJRtx1YSzY3fy9Ch1CEkRo+AOCH6LYhISwbTmwQo=",
"users"=>{"pid"=>"366053", "password"=>"[FILTERED]",
"remember_me"=>"0"}, "commit"=>"Anmelden"}
Completed 401 Unauthorized in 1ms
Processing by Devise::SessionsController#new as HTML
Parameters: {"utf8"=>"✓",
"authenticity_token"=>"dHDnJRtx1YSzY3fy9Ch1CEkRo+AOCH6LYhISwbTmwQo=",
"users"=>{"pid"=>"366053", "password"=>"[FILTERED]",
"remember_me"=>"0"}, "commit"=>"Anmelden"}
Rendered devise/_links.erb (0.8ms)
Rendered devise/sessions/new.html.erb within layouts/application
(29.5ms)
Completed 200 OK in 119ms (Views: 100.9ms | ActiveRecord: 1.7ms)

routes.rb
=====
devise_for :users, :skip => [:sessions]
as :user do
get 'login' => 'devise/sessions#new'
post 'login' => 'devise/sessions#create', :as => :login
delete 'logout' => 'devise/sessions#destroy'
end

authenticated :user do
root :to => 'surveys#index'
end

root :to => 'home#index'

index-page
=====
<%= render 'devise/sessions/new' %>

devise/sessions/_new.html.erb
=====
<%= form_for('users', :url => login_path) do |f| %>
<table>
<tr>
<th><%= f.label :pid %></th>
<td><%= f.text_field :pid %></td>
</tr><tr>
<th><%= f.label :password %></th>
<td><%= f.password_field :password %></td>
</tr><tr>
<td><%= f.check_box :remember_me %> <%= f.label :remember_me %></
td>
<td><%= f.submit "Anmelden" %></td>
</tr>
</table>
<% end %>

config/initializers/devise
=====
Devise.setup do |config|
config.mailer_sender = "nor...@sonicco.de"
config.apply_schema = false
require 'devise/orm/active_record'
config.authentication_keys = [ :pid ]
config.case_insensitive_keys = [ ]
config.strip_whitespace_keys = [ ]
config.skip_session_storage = [:http_auth]
config.stretches = Rails.env.test? ? 1 : 10
config.remember_for = 2.weeks
config.use_salt_as_remember_token = true
config.password_length = 8..128
config.lock_strategy = :failed_attempts
config.unlock_strategy = :none
config.maximum_attempts = 5
config.navigational_formats = [:html, "*/*", :"*/*"]
config.sign_out_via = :delete
end

Any idea to fix the problem? Login via /login works fine for me, but
never by using the partial. Always get redirect to the login form with
401 error.