While your site is publicly accessible, we cannot access your site’s XML-RPC file:
https://zagrebarnica.onlinewebshop.net/xmlrpc.php
This file is used by Jetpack and other plugins and apps to connect to your site. It must return the following message:
XML-RPC server accepts POST requests only.
You can see a working example here:
https://jetpack.com/xmlrpc.php
Since your site returns an error for this page, I would recommend that you check the following:
1. Can you see the xmlrpc.php file at the root of your WordPress installation when accessing your site via FTP?
2. Do you use any security plugins that may block access to this file? If so, could you disable them?
3. Does your hosting provider block access to this file? You may need to contact them to find out.
Let us know how that goes!