Magento – Admin Panel not working

While it may offer you a temporarily solution, you should considering not modifying the core code like that to solve problems. Changing the source code of an application will create problems that are much more difficult to track down.

There's a number of different issues that cause the errorless admin login behavior you're seeing, but they all go back to Magento not being able to set or read the session cookie. Magento uses sessions to pass error messages between pages — that's why you don't see an error message. Magento also uses sessions to store the "is logged in" value, so not setting sessions also causes the core error behavior.

Possible causes include

• Local computer time vs. server time mismatch, causing instant cookie invalidation. Make sure your server time is correct.

• Incorrect permissions on var/session, preventing session files from being saved

• Incorrect configuration of database/redis/other session storage, preventing saving of session values

• A module is instantiating sessions to early, preventing the correct session names from being set

• You're a developer using multiple URLs and have multiple cookie domains

• Another developer has somehow modified app\code\core\Mage\Core\Model\Session\Abstract\Varien.php, creating a hard to track down bug

• The cookie domain in System -> Configuration -> Web -> Session Cookie Management doesn't match the actual site domain.

• You're using the localhost as your server domain, and using a version of webkit that has trouble/bugs setting cookies for localhost in some situations.

The short term fix is to just delete your cookie for the domain. That's often enough to solve the problem. If it persists, figure out which of the above reasons is the reason for your error, and take steps to address it (fix permissions, etc.)

You don't list what you've tried, so it's hard to rule anything out, but here are a few to try:

• Check what type of session management is configured in app/etc/local.xml. If it's files, just delete the contents of the var/session directory. If it's db, then truncate the core_session table. If you're using something more complicated like Redis or memcached, make sure it's actually installed and configured correctly, especially seeing as you say you've moved servers.
• Incorrect permissions on var/session, meaning the server can't write to the directory and sessions can't be started
• Make sure your browser isn't blocking cookies
• Try in private browsing mode, or a different browser
• Check your cookies. Make sure there are no duplicate adminhtml cookies
• Make sure there is no PHPSESSID cookie - this is an indication that the session is being created too early, using the default PHP session cookie name, rather than Magento's choice of name for admin sessions (adminhtml). This could indicate an extension may trying to start a session too early.
• Have you installed any new extensions? Try setting disable_local_modules to true in app/etc/local.xml. If you can now login, try switch it back to false, disable all your community and local modules and then try enabling them one by one.
• Are you hosting the site locally? Set your hostname to something that includes periods (.). Webkit browsers have problems setting cookies to domains without any periods (e.g. http://localhost/).

• Make sure you have the correct cookie domain set, especially if you have multiple websites. Use this SQL snippet to check what's configured:

  select * from core_config_data WHERE path = 'web/cookie/cookie_domain';

• Your cookie domain doesn't match your actual site domain

• Mismatch between server time and local computer time, meaning cookies are instantly unset

• Someone has hacked the class Mage_Core_Model_Session_Abstract_Varien. Download a copy of the Magento version your website is running from the Magento website and compare the class in there to your class.

• Not enough disk space on the server, preventing session files being written

• A developer has removed the formkey form element (unlikely), preventing Magento from processing the form. Look in the HTML source code of the login page for a form field called formkey

• Use n98-magerun's sys:check command to find any other issues which may be causing this behaviour

• There is a login issue in Google Chrome. Set Use HTTP only to No in the Session Cookie Management section of System, Configuration, Web. Don't do this on production servers as it is a security risk. Use this SQL snippet to update it: UPDATE core_config_data SET value = '0' WHERE path = 'web/cookie/cookie_httponly';