Whoops! Your session has expired.
Zencart 1.3.8a
PHP Version: 5.2.8 (Zend: 2.2.0)
Internet Explorer 8 version 8.0.6001.18943

I get the "Whoops! Your session has expired." before the purchase
Internet Explorer 8 version 8.0.6001.18943
Internet Options: Privacy High

when you try to change:
shipping method change or
Coupon refresh

I do not have this problem if Internet Options: is set on "Medium High"

I hope someone can help resolve the non-working "High" setting.