WordPress Bug: You don’t have permission to do that

Symptom: Usually this error appear when I try to add new categories to my blog. Off course, I did as administrator, I tried several times but there in no luck.

Finally, I found solution in word press forum by simply commenting some lines of code in wp-includes/pluggable.php file and adding two lines of code. What you have to do is here.

I replaced lines 244 through 250 of wp-includes/pluggable.php

  $cookie = explode('; ', urldecode(empty($_POST['cookie'])

? $_GET['cookie'] : $_POST['cookie']));

foreach ( $cookie as $tasty )

{

   if ( false !== strpos($tasty, USER_COOKIE) )

 		$user = substr(strstr($tasty, '='), 1);

 	if ( false !== strpos($tasty, PASS_COOKIE) )

 		$pass = substr(strstr($tasty, '='), 1);

 }

With

  $user = $_COOKIE[USER_COOKIE];

  $pass = $_COOKIE[PASS_COOKIE];

More details on this issue can be found here :
http://trac.wordpress.org/ticket/3807