drb error

Processing DeliveryUnitController#create (for 192.168.1.5 at 2009-03-20 17:21:46) [POST]
  Parameters: {"itemname"=>{"contains"=>""}, "action"=>"create", "id"=>"300607680", "order"=>{"id"=>"300607680"}, "delivery_unit"=>{"end_at"=>"2009-03-21", "start_at"=>"2009-03-21", "pv"=>"1", "additional_path"=>"", "item_id"=>"100168"}, "controller"=>"delivery_unit", "search"=>"検索"}

...

Completed in 482ms (View: 1, DB: 30) | 200 OK [http://localhost/so/delivery_unit/create/300607680]/!\ FAILSAFE /!\  Fri Mar 20 17:21:47 +0900 2009
  Status: 500 Internal Server Error  can't dump
    /usr/lib/ruby/1.8/drb/drb.rb:395:in `_dump'
    /usr/lib/ruby/gems/1.8/gems/activerecord-2.3.2/lib/active_record/session_store.rb:67:in `dump'
    /usr/lib/ruby/gems/1.8/gems/activerecord-2.3.2/lib/active_record/session_store.rb:67:in `marshal'
    /usr/lib/ruby/gems/1.8/gems/activerecord-2.3.2/lib/active_record/session_store.rb:123:in `marshal_da
ta!'
    /usr/lib/ruby/gems/1.8/gems/activesupport-2.3.2/lib/active_support/callbacks.rb:178:in `send'