I am having the exact same issue. I am unable to add any items to the cart (I get the same error as above) and I am unable to login (I get the same error as above) in IE6. IE7 and above work just fine.

I am using cart links. Any ideas? This is a major issue and I will not be able to launch the site until this is fixed. This is the last issue, besides some themeing issues that need to be taken care of before I can launch my first Drupal site.