While I am unsure of the helpfulness of this comment, my former project used Netscape Enterprise 3.6/Activestate Perl using HTTPS on NT4. The site used an encrypted cookie login/session management scheme.

This site worked great with both IE and NS. Until the user base upgraded their IE to 5.x, at which point, you could login, but not stay logged in as every POST sent you right back to login, indicating a serious issue with our cookies.

We "solved" the issue by just forcing IE to use HTTP 1.0 in the preferences/internet options (it was defaulting to HTTP 1.1) I have no idea if this was ever resolved (the aparent bug in IE), as we just forced IE users to adjust and declared victory. At the time it only affected a few users and we did not have the resources to do anything more (like debug MS software).

you may want to give this a try, if you haven't already...good luck!