Insecure elements not displaying on page

Hi WPMU DEV!

After some recent updates to my site it seems that insecure elements are not displaying correctly so some pages look a bit of a mess. I have an HTTPS plugin which solved this problem when it happened before but it is no longer doing the job of displaying all elements on a page. When I click a link it now comes up with something like this in the URL bar:

https://veganfitness.com/wp-content/cache/page_enhanced/veganfitness.com/about/_index.html_gzip

Whereas it should simply say:

https://veganfitness.com/about

Firefox is telling me that some unencrypted elements on the page have been blocked. When I choose to display all elements it then renders everything correctly.

Any advice would be really appreciated!

Thanks.
Darren.