Hello, I am also having this problem, the ajax javascript error is 500 (internal server error) when one attempts to check out or create an account.
EDIT: Fixed. The change to ObjectModel proposed here:
https://github.com/PrestaShop/PrestaShop/commit/a043e190daae207bdda1a6a181aee931c3edb4a7
changed the logic of the if else condition and resulted in the values not being saved to $this->{$field}. Correcting the logic error fixed the problem.