The ProblemThe problem will manifest itself as a redirect back to the login screen, even though you typed the right username and password. If this problem is affecting you, you will be redirected back and see no error message. This indicates you have the right credentials, but the Magento Admin is just not letting you in. You can verify it by typing the wrong username and password, you’ll see you get redirected back and it shows an error message.
The problem occurs because the Magento backend tries to set a cookie on your browser, and then for some reason when you next make a request, the cookie is gone(or was never there). This makes Magento think you have never logged in, and of course it redirects you to the login screen. So the real guts of it is the missing cookie, we need to find out why it’s missing.