We are experiencing this same admin page issue.
- Initially had hosts set without www and could access admin without problems.
- Checkout was not redirecting to https so we changed config.php using
www.domainname.com as both http and https hosts.
- Now admin does not allow access past home.php. When user name and login are entered, the page refreshes back to itself (admin/home.php)
We can manually access the https connection and the ssl is valid and set up correctly.
We set up several administrators prior to this issue and have tried all combos so I do not think it is a validation issue. We have made no changes to any htaccess file before or after this issue started.
admin/.htaccess contents:
DirectoryIndex home.php