update: the problem is fixed by just upgrading the Sign module through Apps

Hi,

The issue of the Date field showing "undefined" in Odoo Web Enterprise 17.0+e when using the "Sign and Send" feature is typically caused by a misconfiguration in the field definition, such as the date field missing or mismatched in the model or view XML, or by JavaScript errors affecting field rendering; to resolve this, ensure the date field is properly defined in both the Python model and the XML view, check for and fix any JavaScript errors in the browser console, maintain the correct date format expected by Odoo, and clear both browser and Odoo caches after making adjustments, since these steps address the majority of "undefined" field issues, particularly in custom or recently upgraded installations.

Hope it helps