Unable to proceed through payment gateway (maybe anJavascript validation error)

Hi Guys. I'm having an issue we just can't solve!

The problem is related to a payment gateway, and the page & site in question is located here.

When users input all details required to checkout they are confronted with a field error. If you fill in the form and try to checkout you'll see the same issue. It worked well for a long time and it seems a recent theme update caused the error.

Please help - we've tried everything!

Thanks a lot,