Hello Valentin,

It doesn’t look like you are using the correct form code.

Log into your Mailchimp account, select your List and then select to create a form.

Then select the Naked Form option and copy the form code you see.

Here is a screenshot https://www.dropbox.com/s/pmbrhct1odbmspq/mailchimp-form.PNG?dl=0

I just tried it with my account and it works, but you need to replace it with a form from your account.