Cannot log in to drupal admin in a new installation (access denied)
It may also be a cookie problem. To check if that is what causing it, just try to use another web browser to verify if you get the same problem.

If using that other browser the problem disappaears, then DELETE the cookie in the browser where you experience this problem and try again. Pretty sure your issue will be solved then.

Curious to hear if this helps (sometimes it does ...).