IE 9 by default doesn’t like mixture and ssl and non-ssl content. so you get a popup “Only secure content is displayed” at the bottom of the screen if the mixture occurs. The best solution is to ensure that all content is grabbed from https:// instead of http://
In woocommerce, “forced secure checkout” and “Un-force HTTPS when leaving the checkout” are always a good option to check but things become trickier. For some reason, there is a call to https://yoursite/checkout/:/0 in the checkout page. This page doesn’t exists and so it redirects to “page not found”. Because “Un-force HTTPS when leaving the checkout” is checked, the checkout page (which is https) will grab the http content instead. To fix this, uncheck “Un-force HTTPS when leaving the checkout”.