Rails - Devise - Warden and InvalidAuthenticityToken

Hi

I've just uploaded my new website to my server and, after a couple of
teething problems, seem to be hitting problems when I am using devise
and warden to authenticate users etc.

The following is the error message with sensitive information removed:

Processing RegistrationsController#create (for 81.111.90.194 at
2010-08-19 21:35:43) [POST]
  Parameters: {"commit"=>"Sign up", "action"=>"create",
"authenticity_token"=>"XXXXXXXREMOVEDXXXXXXX",
"controller"=>"registrations", "user"=>{"dob"=>"XXXXXXX",
"password_confirmation"=>"[FILTERED]", "firstname"=>"XXX",
"password"=>"[FILTERED]", "surname"=>"XXX", "email"=>"XXXXXX"}}

ActionController::InvalidAuthenticityToken
(ActionController::InvalidAuthenticityToken):
  warden (0.10.7) [v] lib/warden/manager.rb:35:in `call'
  warden (0.10.7) [v] lib/warden/manager.rb:34:in `catch'
  warden (0.10.7) [v] lib/warden/manager.rb:34:in `call'

Rendering /home/enigmaho/rails_apps/newwt/public/422.html (422
Unprocessable Entity)

I have no real idea what is going wrong. It works OK on my development
server at home, and everything else seems to work barring this.

My environment.rb file has the following lines:

  config.gem 'warden'
  config.gem 'devise'

And they have been installed in my plugins/gems directory using "rake
gems:unpack"

I'm using Rails 2.3.8 if that's any help.

Any help would be excellent!

Darren