lighttpd fails to start fastcgi backend. how can i fix this?

I try to run a rails app on a lighttpd, but the server does not start because the fastcgi-backend fails to start. This is the error log of lighttpd:

2011-08-02 20:43:15: (log.c.166) server started 
2011-08-02 20:43:15: (mod_fastcgi.c.1104) the fastcgi-backend /opt/gemeinschaft/public/dispatch.fcgi failed to start: 
2011-08-02 20:43:15: (mod_fastcgi.c.1108) child exited with status 13 /opt/gemeinschaft/public/dispatch.fcgi 
2011-08-02 20:43:15: (mod_fastcgi.c.1111) If you're trying to run your app as a FastCGI backend, make sure you're using the FastCGI-enabled version.
If this is PHP on Gentoo, add 'fastcgi' to the USE flags. 
2011-08-02 20:43:15: (mod_fastcgi.c.1399) [ERROR]: spawning fcgi failed. 
2011-08-02 20:43:15: (server.c.938) Configuration of plugins failed. Going down.

My lighttpd.conf looks like this:

server.modules = (
    "mod_access",
    "mod_alias",
    "mod_compress",
    "mod_redirect",
    "mod_rewrite",
    "mod_fastcgi",
    "mod_accesslog",
)
server.errorlog            = "/var/log/lighttpd/error.log"
accesslog.filename         = "/var/log/lighttpd/access.log"
server.document-root        = "/var/www"
server.upload-dirs          = ( "/var/cache/lighttpd/uploads" )
server.pid-file             = "/var/run/lighttpd.pid"
server.username             = "www-data"
server.groupname            = "www-data"

index-file.names            = (
                                "index.html",
                                "index.htm",
                              )

url.access-deny             = ( "~", ".inc", ".htaccess", ".htpasswd" )

static-file.exclude-extensions = ( ".fcgi" )

server.port = 80
include_shell "/usr/share/lighttpd/use-ipv6.pl"

dir-listing.encoding        = "utf-8"
server.dir-listing          = "disable"

compress.cache-dir          = "/var/cache/lighttpd/compress/"
compress.filetype           = (
                                "application/x-javascript",
                                "text/css",
                                "text/html",
                                "text/plain",
                              )

server.reject-expect-100-with-417 = "disable"
# Available since 1.4.21.
# http://redmine.lighttpd.net/wiki/lighttpd/Release-1.4.21
# http://redmine.lighttpd.net/issues/1017

include_shell "/usr/share/lighttpd/create-mime.assign.pl"
include_shell "/usr/share/lighttpd/include-conf-enabled.pl"

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$SERVER["socket"] == ":80" {
            #   "^/((?!(setting|freeswitch)).*)" => "https://%1/$1" 
    url.rewrite-once = ("^/(setting|freeswitch|manufacturer_snom).*" => "$0",
            "^/(.*)" => "/secure/$1")

    $HTTP["host"] =~ "(.*)" {
        url.redirect = ("^/secure/(.*)" => "https://%1/$1")
    }
    server.document-root     = "/opt/gemeinschaft/public/"
    server.error-handler-404 = "/dispatch.fcgi"
    fastcgi.server = (
        ".fcgi" => (
            "Gemeinschaft" => (
                "socket" => "/tmp/gemeinschaft-fcgi.socket",
                "bin-path" => "/opt/gemeinschaft/public/dispatch.fcgi",
                "min-procs" => 1,
                "max-procs" => 2,
            )
        )
    )


}

$SERVER["socket"] == ":443" {
    ssl.engine = "enable"
       ssl.pemfile = "/etc/ssl/amooma/server.pem" 

    server.document-root     = "/opt/gemeinschaft/public/"
    server.error-handler-404 = "/dispatch.fcgi"
    fastcgi.server = (
        ".fcgi" => (
            "Gemeinschaft" => (
                "socket" => "/tmp/gemeinschaft-fcgi.socket",
                "bin-path" => "/opt/gemeinschaft/public/dispatch.fcgi",
                "min-procs" => 1,
                "max-procs" => 2
            )
        )
    )
    #fastcgi.debug = 1
}

And here is the dispatch.fcgi:

#!/usr/bin/ruby1.9.1

require 'rubygems'
require 'fcgi'

ENV['RAILS_ENV'] ||= 'development'

# Set GEM_PATH and GEM_HOME
#ENV['GEM_PATH'] ||= '/usr/local/rvm/gems/ruby-1.9.2-p290/gems' #'/usr/local/lib/ruby/gems/1.9.1/gems/'
#ENV['GEM_HOME'] ||= '/usr/local/rvm/gems/ruby-1.9.2-p290/gems' #'/usr/local/lib/ruby/gems/1.9.1/gems/'
# Must not be used, otherwise we get a strange error:
# "Could not find rake-0.8.7 in any of the sources".

#require File.join(File.dirname(__FILE__), '../config/environment')
require File.join('/home/ft/programming/Gemeinschaft4/config/environment')

class Rack::PathInfoRewriter
  def initialize(app)
    @app = app
  end

  def call(env)
    env.delete('SCRIPT_NAME')
    parts = env['REQUEST_URI'].split('?')
    env['PATH_INFO'    ] = parts[0]
    env['QUERY_STRING' ] = parts[1].to_s
    @app.call(env)
  end
end

Rack::Handler::FastCGI.run(
  Rack::PathInfoRewriter.new(
    Gemeinschaft4::Application
  )
)

running the dispatch.fcgi as ruby script like this: ruby1.9.1 /path/to/dispatch.fcgi produces no error.

when there is no bin-path entry in lighttpd.conf the server starts. But whenever i add

"bin-path" => /some/path/dispatch.fcgi

this error occurs again.

Any hint would be appreciated.

I understand I am not answering your question directly, but have you considered using Passenger or Unicorn to run your Rails app? The Passenger (whether via Apache or Nginx) or Unicorn are much more established / production ready application environments.