You have both SmithCart and Smith Registration Pro module installed on the same portal, the message indicates a conflict between the cart customer object and the registration customer object. We have both the cart and reg pro modules installed on our web site and havnt see any issues:
Cart: https://www.smith-consulting.com/buynow.aspx
Reg Pro: https://www.smith-consulting.com/buynow/makeapayment.aspx
So the cart works fine? and the Reg Pro module doesnt work? Can you provide more info about which module doesnt work and steps to recreate the error message.
Thanks,
Scott