I checked the details and can see that form used on page does not exists at site backend.
Currently Web form used on checkout page is 76960 : http://take.ms/iaFAf
The same webform does not exists on site backend : http://take.ms/kSBrr
Registration Layout includes the form which does not exists i, e 76960 : http://take.ms/v0v1k
Thus the error.
So you would need to create the form and then add the form in Registration layout , save it and then test it , it should work.
Thanks,
Sanjit