I finally managed to install certificate for Dovecot via gui and also i must install it for another domain of main being mail.superhost.gr
When i go to Let's Encrypt tab and insert the domain mail.superhost.gr there i get
Requesting a certificate for superhost.gr, www.superhost.gr, mail.superhost.gr, autoconfig.superhost.gr, autodiscover.superhost.gr from Let's Encrypt .. .. request failed : Web-based validation failed : Failed to request certificate : mail.superhost.gr challenge did not pass: Invalid response from http://mail.superhost.gr/.well-known/acme-challenge/ehjlhZ395d10M4mgRL50... [2a02:7b40:b0df:81dc::1]: "\n \n \n \n Error: 404 Not Found</title" DNS-based validation failed : Failed to request certificate : Undefined subroutine &main::restart_zone called at /usr/libexec/webmin/webmin/letsencrypt-dns.pl line 47.
****mail.superhost.gr challenge did not pass: DNS problem: NXDOMAIN looking up TXT for _acme-challenge.mail.superhost.gr****
Could someone please exaplin to me whay this happens and what step i need to take to fix it?