While the problem happens almost always to users in Internet Explorer, it has happened in Firefox and Chrome a few times as well.
The issue is in which users attempt to login and either get a 422 error or are simply re-directed back to the login screen. We have a wildcard cookie, but we recently switched it to targeting www (about 3 months ago) and the previous cookie was set to expire at the end of the session.
We also recently switched to Devise authentication from Restful_athentication in hopes that it would fix the issue, but it doesn't seem to be helping. However, I don't see how these are related as they don't really manage the cookies, Rails does, they just put things in the cookies. I thought maybe the issue was that restful-auth was putting something in that got corrupt under weird conditions, but that apparently was not the case at all
UPDATE
After we switched to Devise, users encountering this problem no longer got a 422 error, but now the page just refreshes and nothing happens. I should also mention that when they enter in their email to retrieve their password (as they think they're entering their password in wrong), it returns that the email is not found in the system.
When they clear their cookies, particularly in IE, they still run into the same problem. I've walked a few users through searching for the individual cookies in Firefox and that has solved the problem. However, simply just going to clear cookies the normal way didn't work. The issue was mainly effecting IE7 users, but seems to be all across the map, with users reporting the issue on Chrome, IE8, Firefox, Safari and Safari for the Ipad.
Also, here's a link to the code for the session_store.rb here and here the host is set in def ensure_domain, which is here
UPDATE 2
I just made a very minor adjustment and it changed it so it always sends a host. I tried logging in every different way on every browser and didn't run into any issues. I'm hoping this helps
UPDATE 3 (LAST UPDATE)
We tried a combination of a few different things, so while I'm not exactly sure what fixed it, my best guess is removing the cache from 4 days to when the session ends/browser closes. While I'm not thrilled about this as I personally dislike having to login every time, I'd rather have the problem fixed (and this issue didn't start occurring until after we changed the cache length to 4 days.)
Sending a host was a mistake and actually made the problem worse for everyone as no one was able to login. Another strange thing was that when looking at the cookies created there were two Session ID cookies, which I would guess were causing the conflict, particularly with IE7. Some users are having to clear their cookies to get logged in still, but at least that seems to be working most of the time.
To any having a similar issue, sorry I couldn't be of more help!