If you encountered the following problem when upgrading from CodeIgniter 1.6.x to 1.7.x, then here is a workaround.

A PHP Error was encountered

Severity: 4096

Message: Object of class stdClass could not be converted to string

Filename: libraries/Session.php

Line Number: 715

A quick fix can be made by overriding Session class. As exposed at this forum thread, you can create MY_Session.php and place it under your libraries folder (When you place MY_ prefix, the class is automatically loaded).