Thank you for your answers. I've been trying to move from the nginx verification to the webroot one.
I'm doing it with sudo certbot certonly --cert-name example.com --force-renewal -a webroot -w /etc/letsencrypt/www
According to this answer, it's correct.
I have not yet modified my web server configuration to serve the webroot directory, so I expect it to not work. Yet... it works. And I don't understand why.
Adding --staging --dry-run leads to a 404 error output in the terminal, which is expected.
Is there a cache of some sort?