Because OpenCRM is set up to run on a secure connection, framed content is blocked by default if it isn't also secure. Examples of this are the SMS log in page or the Web tab.
This is caused by your browser settings.
Internet Explorer will visibly prompt you when it blocks content, you it is easy to unblock.
Both Chrome and Firefox are more subtle, they add a shield icon to the address bar. If you encounter a blank screen instead of framed content, make sure to double check the address bar for a shield icon.
Clicking this will give you the option to unblock the unsecured content and this allows the loading of the page.