After debugging a codeigniter app that were installed into a new development environment, I have started to freak out when seeing white screens with nothing more available.
Look near the top of /index.php for a call to error_reporting() - and make sure it's not changing your php.ini configuration to something else (besides E_ALL).
And since you didn't mention your php.ini configuration, check to ensure you have error_reporting = E_ALL there as well.