Hello,
Here are the full error messages:

Error code: 4401 Error message: Unable to populate data, please check if mandatory value is entered correctly.
I tried updating to WooCommerce 3.1 and the only difference is that the error in the console is now:
Unable to fix malformed JSON checkout.min.js?ver=3.1.0:1
Note that registered users can checkout with no problems. The problem only affects guest checkout.