App crash but nothing in logs

Hello, I have a big problem with an application running in production
mode : the app crashes but there’s nothing in the logs
Setup is Apache + FastCGI - I know there is something wrong with my code
somewhere (crash is not related with the setup)

How can I do to have further informations about the error while staying
in production mode ? Should I run an instance of the application in
development mode but with Webrick instead of Apache ?