Mine hap­pened when I installed the Magento on Local­host (Apache).

Prob­lem:

I had a new instal­la­tion of magento. But I was unable to login as an admin­is­tra­tor. I went to the admin login page, entered cor­rect user­name and pass­word but was redi­rected to the same login page. I could not enter the dash­board page. Error mes­sage is dis­played when I enter wrong user­name or pass­word. But noth­ing is dis­played and I am redi­rected to the same login page when I insert cor­rect user­name and password.

Solu­tion:

I googled and found these solutions:-

1) Use 127.0.0.1 instead of local­host in your url, i.e. using http://127.0.0.1/magento/index.php/admin instead of
http://localhost/magento/index.php/admin . But this didn’t solve my problem.

2) Since I am using Win­dows XP, I was sug­gested to open “host” file from
C:\WINDOWS\system32\drivers\etc and have 127.0.0.1 point to some­thing like magento.localhost or even 127.0.0.1 point to http://www.localhost.com . But this also didn’t work either.

3) This solu­tion finally helped me out of this prob­lem. The solu­tion was to mod­ify the core Magento code. Open app/code/core/Mage/Core/Model/Session/Abstract/Varien.php. Com­ment out the lines 80 to 83. The line num­ber may vary accord­ing to the Magento ver­sion. But these lines are present some­where near line 80.

// set session cookie params
session_set_cookie_params(
$this->getCookie()->getLifetime(),
$this->getCookie()->getPath()//,
//$this->getCookie()->getDomain(),
//$this->getCookie()->isSecure(),
//$this->getCookie()->getHttponly()
);

Update (For Magento 1.4.*)

In Magento 1.4, you have to com­ment code from line 86 to 98 in app/code/core/Mage/Core/Model/Session/Abstract/Varien.php. Like this:

/*  if (!$cookieParams['httponly']) {
	unset($cookieParams['httponly']);
	if (!$cookieParams['secure']) {
		unset($cookieParams['secure']);
		if (!$cookieParams['domain']) {
			unset($cookieParams['domain']);
		}
	}
}

if (isset($cookieParams['domain'])) {
	$cookieParams['domain'] = $cookie->getDomain();
} */