Since the External URL Widget was first implemented, web browser security has become much tighter. As such, you'll find many times now when you simply cannot use the External URL Widget.
There are two popular reasons why it doesn't work at times:
1) Cross-site SSL differences. If CloudTest is served over SSL, you won't be able to point to a non-secure URL.
2) Cross-Origin Security policy that's becoming very common to specifically disallow other websites to create iframes to your content.