[Geschlossen] WPML and Gravity Forms

Background of the issue:
I am trying to let users fill out a form using WPML and Gravity Forms. The issue can be seen on this page: versteckter Link. It works on the english form. I updated the translation and made the field not required, to no avail.

Symptoms:
I expected a successful form submission, but instead, I got an error message: 'Hubo un problema con tu envío. Por favor, revisa los siguientes campos. Selección no válida. Introduce una de las opciones disponibles.' This issue occurs in both the French and Spanish translations.

Questions:
Why am I receiving an error message when submitting the form?
How can I resolve the issue with invalid selection in the form?