Effectively, this was the problem!!
I had this:
127.0.1.1 mydomain_old.com mydomain_old mydomain.com
So: 1) the old domain name, 2) the old hostname, 3) the current domain name (& hostname)
I edited that file:
sudo /etc/hosts
and modified that line to:
127.0.1.1 mydomain.com
Then rebooted the server and then this were the hostname outputs:
hostname
mydomain.com
hostname -f
mydomain.com
Then i run the Hestia command:
v-add-letsencrypt-host
and this time all worked fine (empty output… so no problems). And i reloaded the mydomain.com on browser and voilà! the server is serving a NEW certificate with today emission date 
Notes for other people: the file /etc/hosts can be automatically changed on reboot depending on your VM provider, because they put on your machine some scripts to automatize settings like these in the virtualized machines.
Comment/question for HestiaCP developers:
Would it be a good idea to modify the command v-add-letsencrypt-host to check if the domain to be “certified” is really hosted in this server? Yes, i know that a problem like mine could be very uncommon, but probably i’m not the unique user who has suffered a missconfiguration like this regarding to the hostname. Would be easily check-able this domain being hosted (obviously not looking for the hostname output)?
Anyway, many thanks guys for your tireless effort to help. I’m in debt -more!- to this community. Thanks @eris and @Raphael and @jlguerrero .