This is very common at Dreamhost. It is not logged in the Rails logfiles because Rails doesn't even start. Look into the Dreamhost logfile: ~/logs//http/error.log
The problem is that a new FCGI process is often killed by the server. Dreamhost servers like to kill applications, it's a known issue. I used to get this a lot when I restarted FCGI the brutal way, by killing all dispatch.fcgi processes, after which I get 500s for minutes. To tell FCGI to restart, it's better to "touch public/dispatch.fcgi" instead of "killall -9 dispatch.fcgi".
There's a "Dreamhost plugin" that helps to reduce these 500 errors: http://gabrito.com/post/ruby-on-rails-dreamhost-plugin