How to Change Cookies Lifetime Without Admin Login in Magento 1.8

Ten months later, I'm pretty sure this bug was caused by instantiating a session object too early in Magento's request lifecycle (via an observer). If a session is instantiated too early, the PHP cookie name will be PHPSESSID instead of adminhtml or frontend.

This won't halt system operation, but in certain conditional code paths where a cookie's already been set it might cause a login token set in the session to be improperly read/written, leading to "I can't login" errors.

The full explanation is too long for a single Stack Exchange answer, but you can find it here if you're interested. (self-link)

You need to force Magento to use the same cookie domain in all cases. This solution was tested on Magento 1.9.2.4

When the Session Cookie Management variables are not set incorrectly Magento may create two "frontend" cookies with different cookie domains. This typically happens during the login process when you have HTTPS/SSL in the secure URL. Here are workable values for the Session cookie management configuration.

Cookie Life Time: 3600
Cookie Path: /
Cookie Domain : .mydomain.com (The dot prefix is important)
Use HTTP Only : No
Cookie Restriction Mode: No 

1. In the admin panel go to System -> Configuration -> Web-> Session Cookie Management
2. Set the Cookie Management configuration as shown above
3. Save the configuration and clear the magento cache
4. Restart your web server to clear the PHP variables in case you made the change directly in MYSQL database
5. Clear your browser cache to remove any existing cookies associated with your magento site

Using the dot prefix on the cookie domain is important to prevent duplicate cookies from being created for the secure URL

You can inspect your sites cookies using the developer tools in the FireFox or chrome browser.

Changing the cookie domain will also affect the adminhtml cookie used by the back end. Make sure to clear your browser cache to prevent admin login problems.