この問題に関する質問を最初に serverfault.com に投稿しました: https://serverfault.com/questions/152587/apache-mod-proxy-to-another-server
これはサーバーのセットアップの問題ではなく、Rails アプリケーションの問題であることに気付きました。このアプリケーションを、同じサーバー上で実行している別の機能する Rails アプリとまったく同じようにサーバーごとに設定しました。サーバーを起動すると、mongrel.log は次のようになります。
** Daemonized, any open files are closed. Look at /var/www/osuwebdev/tmp/pids/mongrel.pid and log/mongrel.log for info.
** Starting Mongrel listening at 0.0.0.0:8080
** Starting Rails with production environment...
** Rails loaded.
** Loading any Rails specific GemPlugins
** Signals ready. TERM => stop. USR2 => restart. INT => stop (no restart).
** Rails signals registered. HUP => reload (without restart). It might not work well.
** Mongrel 1.1.5 available at 0.0.0.0:8080
** Writing PID file to /var/www/osuwebdev/tmp/pids/mongrel.pid
ページをロードすると、ログに次のように表示されます。
Wed Jun 30 19:46:10 +0000 2010: Error calling Dispatcher.dispatch #<NoMethodError: undefined method `[]' for nil:NilClass>
/var/lib/gems/1.8/gems/mongrel-1.1.5/bin/../lib/mongrel/cgi.rb:108:in `send_cookies'
/var/lib/gems/1.8/gems/mongrel-1.1.5/bin/../lib/mongrel/cgi.rb:136:in `out'
/var/lib/gems/1.8/gems/mongrel-1.1.5/bin/../lib/mongrel/http_response.rb:65:in `start'
/var/lib/gems/1.8/gems/mongrel-1.1.5/bin/../lib/mongrel/cgi.rb:135:in `out'
/var/lib/gems/1.8/gems/mongrel-1.1.5/bin/../lib/mongrel/rails.rb:81:in `process'
/var/lib/gems/1.8/gems/mongrel-1.1.5/bin/../lib/mongrel.rb:159:in `process_client'
/var/lib/gems/1.8/gems/mongrel-1.1.5/bin/../lib/mongrel.rb:158:in `each'
/var/lib/gems/1.8/gems/mongrel-1.1.5/bin/../lib/mongrel.rb:158:in `process_client'
/var/lib/gems/1.8/gems/mongrel-1.1.5/bin/../lib/mongrel.rb:285:in `run'
/var/lib/gems/1.8/gems/mongrel-1.1.5/bin/../lib/mongrel.rb:285:in `initialize'
/var/lib/gems/1.8/gems/mongrel-1.1.5/bin/../lib/mongrel.rb:285:in `new'
/var/lib/gems/1.8/gems/mongrel-1.1.5/bin/../lib/mongrel.rb:285:in `run'
/var/lib/gems/1.8/gems/mongrel-1.1.5/bin/../lib/mongrel.rb:268:in `initialize'
/var/lib/gems/1.8/gems/mongrel-1.1.5/bin/../lib/mongrel.rb:268:in `new'
/var/lib/gems/1.8/gems/mongrel-1.1.5/bin/../lib/mongrel.rb:268:in `run'
/var/lib/gems/1.8/gems/mongrel-1.1.5/bin/../lib/mongrel/configurator.rb:282:in `run'
/var/lib/gems/1.8/gems/mongrel-1.1.5/bin/../lib/mongrel/configurator.rb:281:in `each'
/var/lib/gems/1.8/gems/mongrel-1.1.5/bin/../lib/mongrel/configurator.rb:281:in `run'
/var/lib/gems/1.8/gems/mongrel-1.1.5/bin/mongrel_rails:128:in `run'
/var/lib/gems/1.8/gems/mongrel-1.1.5/bin/../lib/mongrel/command.rb:212:in `run'
/var/lib/gems/1.8/gems/mongrel-1.1.5/bin/mongrel_rails:281
/usr/local/bin/mongrel_rails:19:in `load'
/usr/local/bin/mongrel_rails:19
この問題の原因となるこのアプリの違いがわかりません。