I have this same error. I am trying to use SSL only in the admin area (don't need it in catalogue).
My host does support SSL and I have a Key & Certificate in place on the server.
My admin configure.php file has the following...

Code:
define('HTTP_SERVER', 'http://www.myserver');
  define('HTTPS_SERVER', 'https://www.myserver');
  define('HTTP_CATALOG_SERVER', 'http://www.myserver');
  define('HTTPS_CATALOG_SERVER', 'https://www.myserver');

  // Use secure webserver for catalog module and/or admin areas?
  define('ENABLE_SSL_CATALOG', 'false');
  define('ENABLE_SSL_ADMIN', 'true');
HELP please... any suggestions welcome