[eluser]Peter Korsuize[/eluser]
My two cents (after wasting a few hours trying to log in using IE):
With regards to codeigniter cookies, IE7 does not like underscores in DOMAIN NAMES, as reported earlier in this thread. The underscore in the cookie name ci_session does not appear to be a problem.
Fortunately I only had an underscore in my test setup domain name. But I should have known better. Normally speaking you won't be allowed to register a domain name with an underscore in it to my knowledge.
But it was a good excuse to upgrade from CI 1.61 to 1.71

in my attempts to kill this dragon.