This issue appears when setting “Use Secure URLs in Frontend” to Yes.
Here is the flow to reproduce for me:
1. add a product to the shopping cart
2. click on “proceed to checkout”
Expected result: https checkout page
Actual result: empty shopping cart page
The issue definitely is tied somehow to switching between http and https. As long as I stay on one or the other I’m fine, but as soon as I switch, my cart is emptied.
If I turn off Use Secure URLs in Frontend, or if I force secure and unsecure base urls to be https everything works.
Any ideas? I wanted to roll the site live tonight, but this has to be fixed first.