This is a client side message most likely caused by a reference in your HTML to a non-secured item.
All references in the secure page need to point to “https://”. The easiest way to accomplish this is to use relative links instead of absolute links. Commonly overlooked items include:
Objects (such as Flash, Shockwave,…)
Any of these HTML elements can trigger the warning message. If you are using resources from a third party, you may need to contact them for instructions on using their product on a secured web page.
IFrames can also cause this message. For more information on the IFrame issue follow this link: