Rails application getting 500 error page on production

I'm migrating a working Rails 3 application from server to another type of service. Firstly I had issues with static files (e.g. application.css, application.js) not being rendered (404 status). I resolved that issue turning the config.assets.enabled to true.

Now I'm having the problem of error 500 no matter where I go in the application. I assume it's an asset problem. I don't get an error in log/production. Only something as follow:

Started GET "/" for 187.39.38.147 at 2016-06-14 12:33:05 +0000
Processing by BooksController#index as HTML
  Rendered clubedeautores/layouts/500.html (0.7ms)
Completed 404 Not Found in 16ms (Views: 10.0ms | ActiveRecord: 1.9ms)

I have no idea of how to find out what's wrong to fix that. What can I do to find out what's wrong ?