Mryntu

It also happens if the cookie domain in System > Configuration > Web > Cookies is different from the actual site domain.

To remove the setting without access to the admin panel:

1. 
   

   use the following SQL query on the MySQL console or in a client like phpMyAdmin:

   
   
   

   DELETE FROM core_config_data WHERE path='web/cookie/cookie_domain';
   

   
   



2. clear the cache. If you are using the default file based cache backend, delete all directories below var/cache.

Then as soon as you can log in again, set the correct cookie domain for each website or store (Example: .example.com for example.com, www.example.com and all other subdomains)

If you are using n98-magerun (and you should!), the same can be accomplished with:

n98-magerun config:delete web/cookie/cookie_domain
n98-magerun cache:clean config

There are 3 solutions:

1. 
   

   Use these commands in phpmyadmin

   
   
   

   DELETE FROM core_config_data WHERE path='web/cookie/cookie_domain';

   
   
   

   DELETE FROM core_config_data WHERE path='web/cookie/cookie_path';

   
   

Now Try to login.

2.Delete everything in var folder and then check if it works.

3.Replace the .htaccess file with sample .htaccess file and then try to login the admin.

Hope it help you.

Another possible, somewhat obvious, problem to check first: if your site uses SSL, make sure you're not using the http protocol for your admin page; you should be using https. e.g. https://example.com/admin

In my case the problem seems to be because I've created the admin user using n98-magerun with an user that cannot write on magentofolder/var and used the fallback folder /tmp/magento instead.

I just deleted my admin user and executed a sudo -iu OTHERUSER(user that owns the magento folder) and ran n98-magerun admin:user:create again to create my user.

UPDATE: In another case, the admin url has being visited without www. syntax, and the cookie settings was using www. Just putting the www. in the admin url solved the problem. ;)

Along with the above step of clearing cache, i also had to follow the below article and set the session data information in correct path by following the below steps

Ref article

https://stackoverflow.com/questions/26123081/failed-to-write-session-data-magento

I fixed it by changing the session.save_path to place it in the VM.

Change the file app/etc/local.xml

replaced with below

Then it started working. Also at times you cannot really tell the issue, thus it is important you enable error logging. Enable this by referring to following article

https://www.thecreativedev.com/how-to-enable-system-log-and-errorswarning-in-magento/

Clear cookies and cache. Open admin panel in incognito mode.

This is known issue generally comes after applying patch SUPEE-7405 included in Magento 1.9.2.3 release. It can be solved by adding below code in file - app/code/local/Mage/Core/Model/Session.php

public function validateFormKey()
 $formKey != $this->getFormKey()) 
 return false;
 
 return true;

This file might not exist in your repository so copy it from app/code/core/Mage/Core/Model/Session.php and paste it in app/code/local/Mage/Core/Model/Session.php After that add above function in file because it may be missing in core file.

Also clear your browser cache and cookies. Clear all files in Magento var/cache and var/session folders contents. Then login to your admin panel.