I've had security/login probs the past couple days, all fixed now. Here's a summary of my fixes. I Hope some of this helps you!

I had the problems listed in this thread (but the fix there did not work for me):
http://www.zen-cart.com/forum/showth...+Cart+is+empty

I removed Simple SEO and installed Ultimate SEO. That fixed the shopping cart, new user registration and contact us problems. But existing users still could not login. Here is my fix for that:

When an existing user tried to login the error msg said
ERROR_SECURITY_ERROR

I discovered that my template was missing the login timeout file that provides the security token. I copied all 3 login files from the zen classic directory and login works fine now (just copying the missing one didn't work).

If you're missing any of these files (tpl_time_out_default.php, tpl_login_default.php , tpl_logoff_default.php) copy all 3 from Classic template_default to your template dir.

Source dir is
\zen-cart-v1.3.8a-full-fileset-12112007\includes\templates\template_default\templates

Copy to:
store root\includes\templates\Your_Template_Name

Good Luck
--Pat