Symptoms
When adding certain websites to display via the HTML Widget will not display any content. When viewing the ECC logs you will see errors similar to:
com.uc4.webui.vaadin.framework.dashboard.widgets.iframe.IFrameWidgetController - can not retrieve content/response code server-side for given URL:
Cause
The reason for this behaviour is that the websites are designed to not allow for their content to be embedded.
Resolution
There is no workaround for this issue at this time - this is known and is being reviewed for future versions of the ECC.