Access Denied after Login (exact copy of working site)

I have a live, functional Drupal site that I'm trying to migrate to a new host. After copying the site and database, the site is functional, but when I log in as admin (or any user), I immediately get an access denied page.

This problem occurs even if I copy the site to another user on the same host, and set it to connect to the same live database that the original site is using!

It appears that the user is initially authenticated, but is somehow logged out. I've tried clearing the sessions table without success.

I've also tried copying the site to a local machine with the same problem.

Any ideas?