There are lots of fixes out there that sort this issue out including changing cookie_domain, changing the Admin users password in PHPMyAdmin or editing Varien.php to comment out the session code. Lately though, with the most recent versions of Magento 1 from around v126.96.36.199, this has become a bit of a problem again. Where old solutions used to work on older versions of Magento, the old solutions are not working anymore.
So, if you’re getting this error logging into your Magento 188.8.131.52 or 184.108.40.206 Admin, try this solution when none of the others will work.
Basically, it’s as if the admin users have become corrupt or locked (like what happens in Magento 2). There’s no way to unlock Admin users in Magento 1 so the way around this is to rename your app/code/etc/local.xml to local.xml_bak and open your site again. You should see the Magento Installer appear. Install Magento again by going through the installation process and when you get to the page that asks you to add a new user, add a new user with new credentials and a new email that hasn’t been used before. Complete the installation and open your site again. Try logging into your Admin with the new user details. It will login successfully.
Old users will still see this error appearing so deleting them and adding them again should fix that.
You can perform the Magento installation again on any live Magento store without overwriting any files or database data so don’t worry about deleting or overwriting anything. But at the same time, always make a backup of all files and database before trying this!