Production: no relevant logs to fix this problem

I'm moving my application to production.
There's some cases where I get:

Application error
Rails application failed to start properly

But the production log shows no problems:

That's probably your front-end web server (Apache?) serving up
public/500.html. That can indicate that proxying to your Mongrel or
WEBrick server isn't set up properly.

That's probably your front-end web server (Apache?) serving up
public/500.html. That can indicate that proxying to your Mongrel or
WEBrick server isn't set up properly.

Apache is used. The funny thing is that it happens only in some parts of
the app. The previous one was in a view that was rendering a partial and
I had to remove some parts. But since there is no logged info to
check...

Thanks.

Now it is solved.

I checked the error logs in the Cpanel of the hosting provider I use.

There was "malformed header from script. Bad header=planet:
dispatch.cgi, referer:..."

Googling around I found someone advicing to remove any "puts" in the
controller.
That planet you see in the bad header is actually a puts params[:answer]

I took away the puts and now it goes ok.

Cheers.