When the user submits the form successfully for the first time, a new Shopify customer with above info is created as expected. The problem happens when the user submits the form with an already registred email. In that case, after the submit, he is redirected to the same page and there are no errors in Liquid object "form.errors" (but the Liquid object "form.posted_successfully?"is false). I already tried to remove the "return_to" parameter from the tag "form", but the problem remains the same.
Do you have any idea of the cause of this problem? And how can I show to the user an error message like "This email address has been already taken"?