Lets Encrypt not remembering checkbox setting...

I created a new Site checking the "Let's Encrypt SS: [x]" setting under the Domain tab.
Redirect tab had "Rewrite HTTP to HTTPS: [x]" checked.
Options tab has the following entries for WordPress support:
Custom php.ini settings:

display_errors = On
post_max_size = 99M
upload_max_filesize = 255M
max_execution_time = 220
memory_limit = 256M
max_input_vars = 2000​

nginx Directives:

client_max_body_size 100M;
location / {
try_files $uri $uri/ /index.php?$args;
}
# Add trailing slash to */wp-admin requests.
rewrite /wp-admin$ $scheme://$host$uri/ permanent;
location ~* \.(jpg|jpeg|png|gif|css|js|ico)$ {
expires max;
log_not_found off;
}​

PROBLEM:
After clicking [Save] and waiting for the red circle notification at the top to disappear, if I go back into the Domain tabe for this Site, the Let's Encrypt checkbox is UNCHECKED. I try checking and saving but the LE setting is NOT remembered.
I need to make this a LE secured site so I can install WordPress using HttpS.
Please help to resolve the above problem. Thank you.

Server Info: Ubuntu 16.04, ISPconfig 3.1.13 (just upgraded from 3.1.12 today using default values in update script); nginx web server. I can view the new Site's default page ("Welcome to your website!") when using http://learn.NewSite.net/index.html

https://www.screencast.com/t/TpU7mnC7Jy (screen shot shows result in browser)