This is really does not seem to be a mod_cluster issue, there is nothing modcluster can do about it since the request is already malformed coming from the client (missing session cookie).
I remember, there was a same origin problem in Internet Explorer, i.e. your request need to be relative rather than absolute (e.g. instead of "http://example.com/service" just make ajax call on "/service").
Otherwise just google around for JQuery known issues/problems or maybe try direct ajax call instead http://docs.jquery.com/Ajax/jQuery.ajax#options
Thanks for the reply.
I've tried searching around but didn't find anything yet that is related to my issue.
I thought it might have been mod_cluster because if i run things directly from JBoss Server things work fine. I'm using relative URLs. This only fails when i go through mod_cluster.
Ok, definitely it isn't mod_cluster - the issue turned out to be a naming of my host machine. Apparently IE 8 and possibly other versions of IE doesn't like or doesn't handle them too well when an underscore is in the hostname.
This article helped -
Well, you should really follow RFCs, its not that IE is doing it wrong.