Pulling up a search gave lots of ideas on how to fix this:
1) Use another browser
2) Disable javascript in your browser
3) create an administrator account in your WordPress install, change the original admin account “User” instead of “administrator” privileges, log out and log in under the new account. Give your original admin account back the admin privileges. Log out and back in as admin.
4) Go to your Users area, change your account settings from “administrator” to “User” – then change it back without logging out.
5) Disable your plugins. If you can add a category, activate them one-by-one to see which one is the culprit.
6) Clear your browser’s cache and cookies, restart said browser.
7) A really REALLY tricky solution – it seems to work initially, but no promises – wp-includes/pluggable.php, lines 244 through 251, replace
$cookie = explode('; ', urldecode(empty($_POST['cookie']) ? $_GET['cookie'] : $_POST['cookie'])); // AJAX scripts must pass cookie=document.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];`
(probably be an *excellent* idea to comment out the lines and add the new ones in, rather than delete and replace – in case this solution messes up anything majorly, you can always switch it back.)
Anyway, these are only a *few* solutions I found when searching the forums for this issue. There were tons more.