このエラーが発生しましたが、デバッグ方法がわかりません。コントローラーの作成アクションの「保存」メソッドで発生するようです。
vestal_versions をデプロイしたときに問題が発生し始めたと思います。
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.8/lib/active_record/base.rb:2906:in `assign_attributes'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.8/lib/active_record/base.rb:2902:in `each'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.8/lib/active_record/base.rb:2902:in `assign_attributes'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.8/lib/active_record/base.rb:2775:in `attributes='
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.8/lib/active_record/base.rb:2473:in `initialize'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.8/lib/active_record/reflection.rb:162:in `new'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.8/lib/active_record/reflection.rb:162:in `build_association'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.8/lib/active_record/associations/association_collection.rb:425:in `create_record'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.8/lib/active_record/base.rb:2178:in `with_scope'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.8/lib/active_record/associations/association_collection.rb:424:in `send'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.8/lib/active_record/associations/association_collection.rb:424:in `create_record'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.8/lib/active_record/associations/association_collection.rb:245:in `create'
/home/timothy/.bundler/ruby/1.8/vestal_versions-2773ee610767/lib/vestal_versions/creation.rb:43:in `create_version'