After I enter my username and password and click "Login" it sends me back to the Login page with NO error messages. The URL changes to include a session key and that's it. Everytime I click Login again, the session key is regenerated and I still can't log into the admin interface.
Clear the contents of the directory var/cache
and try again.
If that doesn't help check the table core_config_data
in the database for records with a path
value of "web/secure/base_url", "web/unsecure/base_url" and "admin/url/custom". These should match the page you are trying to log into (but without "index.php/admin/" at the end)
Finally check your browser's cookie settings and that the server's PHP is able to set cookies.