I manually installed an existing live site locally on my computer and was unable to log into the admin backend. when I would click the login button, I wouldn’t get an “invalid login” message or anything b/c it was correct. The URL would change to admin/index/index/key/xxxx and would just display the login form again.
I worked at it for hours and tried all the code changes in the Session object to no avail. I finally installed a fresh copy of magento locally and used their installer and I was able to log in. I noticed it was using the file system for sessions while the other existing site was using the database because it was being load balanced. I changed it to use the filesystem and that fixed the problem! Hope this will save somebody the many hours I spent on it.