My experience is that this site is not robust and does minimal field checking, and consequently blows up rather than reporting a specific error.
For example, my password manager autofilled my zipcode in the Zip+4 field (i.e., put 5 numbers in a field that should only hold 4). When I submitted the form, it blew up with the error message above, instead of simply a) limiting the field to 4 digits, and/or b) reporting that the value wasn't valid.
So if you are getting this error, check your form carefully to make sure you're not inadvertently submitting something invalid.
