I have the same issue
I am also experiencing this issue, the my site was accepting payments from the US but has recently started generating the same error Sage Pay have advised my client the problem is with the script we are using on the site.
I posted a new thread as I could not find this post last night entitled 'SagePay error affecting US customers' I have attached my files, I assume the problem is the values being submitted for the Delivery and Billing State fields, as the site is live I could do with some guidance on this.
I look forward to any advise you can give.
Thanks in advance