Can't get double optin to work

Hi,

I'm trying to activate double-optin using a template derivated from the standard double optin confirmation email.
I can't get it to work as expected. The right email is sent, but the link associated to the validation button results in a 404 error.

Example here : https://www.les-caue-occitanie.fr/newsletter/test

I'm not sure if something is missing in the form itself (some hidden hash) or if it is a SendinBlue issue.

btw: the Drupal module is not (no longer) supported by the SendinBlue :-(

Thnaks in advance for any help on this.
Marc