Hi, I am also seeing exactly the same bug.
I recently moved my site to a new host, Hosting24, moving away from whatever host the original website design company used, and also setup SSL certificate for the checkout.
Somewhere around that time this problem started happening.
Everything else works fine.
It worries me that I may have done something to start this happening.
WordPress version 3.3.1
e-Commerce plugin version 3.8.7.2
Not updated either of those for some time and am resisting updating until I have documented some source code changes I made.
I am sure, therefore, the server error was not introduced by updates.