@Plesk Team:
This is solved, can be marked as "already implemented". :)

@admin: You also need to set the default site to that hostname in :8443/admin/ip-address/edit/id/1, otherwise plesk will still use its own cert.

@eugene: then use <your own domain name>:8443 and set the default site as above

Even when I try the suggestion from Plesk Staff I get this error also:Fout: Het installeren van het Let's Encrypt SSL-certificaat is mislukt: Failed letsencrypt execution: Saving debug log to /usr/local/psa/var/modules/letsencrypt/logs/letsencrypt.log
Starting new HTTPS connection (1): acme-v01.api.letsencrypt.org
Obtaining a new certificate
Performing the following challenges:
http-01 challenge for h2436691.stratoserver.net
http-01 challenge for www.h2436691.stratoserver.net
Starting new HTTPS connection (1): 127.0.0.1
Waiting for verification...
Cleaning up challenges
Generating key (2048 bits): /usr/local/psa/var/modules/letsencrypt/etc/keys/0004_key-certbot.pem
Creating CSR: /usr/local/psa/var/modules/letsencrypt/etc/csr/0004_csr-certbot.pem
An unexpected error occurred:
There were too many requests of a given type :: Error creating new cert :: Too many certificates already issued for: stratoserver.net
Please see the logfiles in /usr/local/psa/var/modules/letsencrypt/logs for more details.

At this moment I am using the last version of Plesk Onyx with Security Advisor. This option has the posibility to secure Plesk with a valid SSL. I was glad to see this option, makes life easy feature I call it. But it keeps giving me an error about the installed certificates been exceeded. But I am sure I do not have that much certificates. Only one with a green bar en two of Lets encrypt. I do not understand how this would give me a problem installing the SSL for Plesk.
This should be a good feature if it realy works. Working on a Linux VPS server.
I get this error:
Fout: Execution letsencrypt-hostname.sh has failed with exit code 1, stdout: , stderr: Saving debug log to /usr/local/psa/var/modules/letsencrypt/root/logs/letsencrypt.log
Starting new HTTPS connection (1): acme-v01.api.letsencrypt.org
Obtaining a new certificate
Performing the following challenges:
http-01 challenge for h2436691.stratoserver.net
Using the webroot path /var/www/vhosts/default/htdocs for all unmatched domains.
Waiting for verification...
Cleaning up challenges
Generating key (2048 bits): /usr/local/psa/var/modules/letsencrypt/root/etc/keys/0009_key-certbot.pem
Creating CSR: /usr/local/psa/var/modules/letsencrypt/root/etc/csr/0009_csr-certbot.pem
An unexpected error occurred:
There were too many requests of a given type :: Error creating new cert :: Too many certificates already issued for: stratoserver.net
Please see the logfiles in /usr/local/psa/var/modules/letsencrypt/root/logs for more details.

Since Startcom/StartSSL is going down the bad way, we need a replacement asap. Let's Encrypt solves this generally, but it's not yet possible to manage multi-subdomain certs in plesk.

Please make it possible to add multiple subdomains to create the mailserver certificate.
Either let the admin choose on their own or define at least the follwoing default subdomains to be included into the host certificate.

mail.my-servername.de
pop.my-servername.de
pop3.my-servername.de
imap.my-servername.de
imap4.my-servername.de
smtp.my-servername.de

Thanks!

Thanks! I just installed it and its working great!

Fred