Comment submit hangs on "Saving"

Page: https://www.doc-ralf.de/bastelbude/blogs/gruenes-witten/98-stickoxid-belastung-in-der-husemannstrasse#!/ccomment

Moderator etc. everything disabled, but this still does not work.

Developer console tells me:
SEC7120: [CORS] Der Ursprung "https://www.doc-ralf.de" konnte "https://www.doc-ralf.de" im Access-Control-Allow-Origin response header für die cross-origin -Ressource bei "https://doc-ralf.de/?option=com_comment&task=comments.getcomments&format=json&lang=en&contentid=98&component=com_content&start=1" nicht finden.

This error only occurs when I want to add a comment. So I think, this has something to do with the https configuration of my homepage. Joomla is set to force https, and configuration.php contains

"public $live_site = 'https://doc-ralf.de';"

Also http accesses are forced to use https in .htaccess file:

RewriteEngine On
RewriteCond %{HTTPS} OFF
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI}

But these are the settings suggested by my homepage provider.

Any suggestion?

Regards Ralf