This looks like the result of a restriction on your server.

Please send me a login - https://www.joomlacontenteditor.net/contact/site-login

Logging into Joomla (as admin)

showing an error message regarding external content and user rights

This is only happening in the front-end, which suggests some kind of redirect or filter / firewall in place, perhaps through your hosting provider. Accepting the messge does load the content in the Preview.

Could this most likely be due to the server, security settings in the admin tools, the implementation of the cookie consent management system, or the forum software jForum?

It appears to be the Cookie Consent Manager. The url of the page showing the message is - https://5f3c395.ccm19.de

The css styling of the form post in media/com_cjforum/css/site.css is removing the list style:

.cjforum-container ul, .cjforum-container ol, .cjforum-container li {
  list-style: none;
  margin: 0;
  padding: 0;
}