Sorry. A non-recoverable error has occurred. Could not find valid Key

I'm running Drupal 6.12 and originally CiviCRM 2.2.6, but just tried with 2.2.7 and can not seem to get CiviEvent anonymous user registration working. I've gone through the issue queue and insured that I have a user 0 in the users table, that sessions table is UTF 8, and that caching is turned off. However, when an anonymous user tries to register, they constantly get this error.

If I am an authenticated user, I can login with no problem.

Any assistance in troubleshooting, or working through this issue further would be much appreciated.

backTrace

.../sites/all/modules/civicrm/CRM/Core/Error.php, backtrace, 258
.../sites/all/modules/civicrm/CRM/Core/Controller.php, fatal, 207
.../sites/all/modules/civicrm/CRM/Core/Controller.php, key, 145
.../sites/all/modules/civicrm/CRM/Event/Controller/Registration.php, __construct, 44
.../sites/all/modules/civicrm/CRM/Core/Invoke.php(201) : eval()'d code, __construct, 1
.../sites/all/modules/civicrm/CRM/Core/Invoke.php, eval, 201
.../sites/all/modules/civicrm/drupal/civicrm.module, invoke, 342
, civicrm_invoke,
.../includes/menu.inc, call_user_func_array, 348
.../index.php, menu_execute_active_handler, 18

Sorry. A non-recoverable error has occurred.
Could not find valid Key

Return to home page.

Thanks,

Tracy