Debug a CodeIgniter website

We are actually looking for a good programmer with CodeIgniter who can fix our differents bugs after an attempt of the core system's upgrade. We got a project working with Code Igniter 2.2.1 and we have tried to upgrade the core to the new version of Code Igniter. I mean the 3.0.6. We have followed the official step. So we have done this step so far : Update the system files of code igniter with the new version. Update our class name files with the new convention. Replace the config/[login to view URL] by the new one Change the structure of the database for following the new directive Replace our error template Change some functions for fitting to the system core. But after all of that, some functionnalities of the website are not working anymore like the backend, the connection to the backend and almost everything related to the database like the creation of new trip by the customers. For the backend, there are many bugs : - We cannot access to the folder - We cannot use any functions "new" - The "edit" functions dont work For the connection to the database, the bug is strange and seems to be close of the connection to the session. I'm almost sure of that but I cannot figure it out how to fix that bug : - We cannot connected to the backend directly If we try to connect to the backend, it's not gonna work. If we try directly after the first try, it's still not gonna work. But if we try after ten minutes, we can access to the backend. There is also one big problem on the front-end : - It's impossible to create a new trip for the customers. we got an error related to the database. So if you are very good in debugging and have a good experience of CodeIgniter, MYSQEL and "sessions", you have a chance to get the job. Thanks