Dear Ross,
Thank you for your interest in CS-Cart shopping cart software.
This problem caused by the fact your web-server forms cookies incorrectly due to some reasons.
I have attached archive with three PHP files inside to this post. Please upload it to the /core/ folder of your CS-Cart installation. This should fix the issue.
Feel free to contact me if you have any other questions concerning CS-Cart.
I’m always glad to help.
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Thank you
Vladimir Kalynyak