I have NO idea why this thread is marked as resolved... none of the fixes here have solved the problem for more than one or two people.
WooThemes does not support WooCommerce as it is a free plugin, so if you submit a ticket it will go unanswered. They only support paying customers, so you will need to purchase an extension or something from them in order to receive support. That said, their support is dismal, documentation is limited and not very useful and with so many of these bugs you'd think /someone/ would be responding. I have the problem as well - customers trying to place orders are getting this error, however I cannot replicate the problem on any browser or address I try.
I'm baffled.