Ok, these errors are not good. They didn’t tell much about what happended.
But you can’t do as I did. Don’t wander on the dark side of the system…
Just try to execute your FCGI as your lighttpd did.
Find your lighttpd user in lighttpd configuration file (debian: /etc/lighttpd/lighttpd.conf).
On my configuration I see:
server.username  = “www-data”

so…

$ su
# cd /my_rails_root/public
# sudo -u www-data ./dispatch.fcgi

Read what’s happen and solve.
That’s all!