I have an existing app built on CodeIgniter 1.5.4, as far as I can tell. It worked fine on PHP 4x. But I just upgraded PHP on my server to 5.1.6 and the app. now presents a blank screen whenever I click a link to a page it used to display, including the control panel log in link. The app uses a MySQL database.
I have tried checking error logs, but have not been able to identify any related error messages. I also have Expression Engine 2 and X-cart on this site and they work fine. At this time I only want to get the app debugged and running again.