Problem with htaccess - cgi e fast cgi

My ROR web application on my webhosting needs to have the .htaccess with
the instruction:
RewriteRule ^(.*)$ dispatch.cgi [QSA,L]

that uses the script dispatch.cgi and that don’t give any error.

Instead if I use, in the htaccess, the rule:
RewriteRule ^(.*)$ dispatch.fcgi [QSA,L]

to call the fast cgi script the application runs more fast and correctly
but generate the log file that is attacched in the message.

I would like to know what is the problem with the fast cgi script to
correct it and to use it because the ROR application is more fast.

Thank you very much.