Issue
External web page is not displaying in Content Control when running a K2 form with the control.
If you are using Chrome, when you open the browser's developer console and check the console, the following error message is shown:
Mixed Content: The site at 'https://<your k2 site>/' was loaded over a secure connection, but the file at 'http://<external site>/...' was loaded over an insecure connection. This file should be served over HTTPS. This download has been blocked.
Resolution
As your K2 site is using HTTPS, modern browsers will expect everything loaded by your K2 form to be using HTTPS as well. If you try to load another page within your K2 form using HTTP, the browser may block them from being displayed.
The recommended solution is to make sure the page you are trying to load in the Content Control is using HTTPS as well. This is the most secure solution.
If for some reason you cannot configure the page to run with HTTPS and you need the page to be loaded inside your K2 form, then you have to reconfigure your K2 site to run using HTTP, such that your K2 form and external page are all running with HTTP. This is a less secure option, so please be aware of the risks if you decide to choose this option.