@Idez Thanks! I know the difference between A and CNAME, but I'm not an expert like you... 
The log error is
acme: error presenting token: gandiv5: no subdomain because the domain and the zone are identical:
My zone file is "typical". Why is this not a problem for other people - is it maybe because of Gandi specifically? I had similar config with other dns providers (e.g. digitalocean, ovh, linode) but never had this weird problem.
I wonder whether the problem is the provider, or is it me (my config).
(This is not a problem, I am using that setting and it works. I'm just trying to understand because there was no docs about this, so I'm curious and it took a long time to figure out.)