Creating new web site based on old CI version

Apache vs. nginx is just going to be a matter of getting the server configured correctly. Unfortunately, I can't be of much help with that, but I'm sure there are some examples out there of how to configure nginx and CI to work together properly.

Honestly, I would probably start with comparing the CI config files you're using to the default config files for that version of CI to see what was changed for your application/server environment. You may also want to check the index.php file to make sure your ENVIRONMENT is set to 'development' while you're working on the upgrade (assuming this is a development server), because you could be suppressing error messages that might help you fix the problem(s).

Bonfire

Practical CodeIgniter 3
CodeIgniter Testing Guide