I had one hell of a time when I copied my Ruby on Rails application from my development machine (Windows XP, Apache 2.2) to my production machine (Linux, Apache 1.3). I floundered through a series of errors and problems, but one error really stumped me, and unfortunately I couldn't find much help on the net for it:
Application error (Rails)
It took me forever to get past this problem, but I finally figured it out. The first line of a few files in the public/ folder contained a path to the Ruby binaries.
#!C:/ruby/bin
For obvious reasons, this wasn't going to fly well on a *nix server. So I had to change it in the following files: dispatch.cgi, dispatch.fcgi, dispatch.rb.
In my instance, it was changed to:
#!/usr/local/bin/ruby
But obviously your server might be different. Try exploring /usr/local and /usr/bin or using locate ruby to try to find the correct path to Ruby.
After I made this modification the application worked fine.