Troubleshooting: My iFrame is not displayed

I have inserted an iFrame into a text but it does not load external content and shows only the placeholder.
What is causing the problem?

To display external content the following requirements are to be met:

The Serviceware Knowledge (SABIO) server must have access to the respective website.
Serviceware Knowledge (SABIO) works only via the https-protocol. To display external content it must be accessed via https as well.

If any element on the external site is integrated via http, the browser will deny access to the complete site for security reasons.
Some websites deny being displayed in an iFrame at all.
If the integration via iFrame does not work at all, consider linking it with an external link in a new browser tab.