The problem was with the login system that I built for an application. Everything was working fine  initially and suddenly for some unknown reason IE 8 decided it didn’t like something and wouldn’t allow people to log in.

After trawling through the CI forums and changing the ci_session to cisession and updating the session length in a vane attempt to resolve the issue I finally found a solution that worked for me.

http://www.philsbury.co.uk/blog/code-igniter-sessions