General Script Error - cart expired

[Kermy]

We have recently moved to a dedicated server but a customer has reported a problem when checking out. After payment authorisation a general script error occurs:

Error: Your shopping cart has expired, orders must be completed within 3 hours or they will be automatically cancelled.

I've looked at this thread but I'm fairly sure there's no issue with the server time. We aren't running SSL, just using Protx for CC details. Is there any other reason this would happen?

[cdicken]

It could be an overactive firewall on the customer's PC deleting the cart cookie after the transfer to Protx. That way, when the customer is passed back, Actinic cannot find the cookie on the customer's PC that corresponds to the store session - hence the error.