Error 500 - Kobo matrix


I am currently trying to incorporate a matrix in one of my surveys. To do so I first started with the basic structure presented here : Question Matrix Response Type — KoBoToolbox documentation
However, I always end up with the Error 500 message.

Could you please help me?

Welcome to the community, @corentinlicoppe1! Have you tried building a matrix question through a form builder UI?

The form builder is the one direclty in my browser on the kobotoolbox interface?

KoBoToolbox UI.