OK I've tried a few things and one of the things that I failed to mention is that I have no sidebar on the forum list. Using Widget framework I cleared it as it is on the CTA featured threads page.
When I removed the 'Clear Sidebar' widget and the sidebar returned the media was showing - weird! Is there any way to correct this using CSS or a template edit as I really don't want the sidebar on the forums list?
This seems to suggest the same thing; it is only a problem when Widget Framework is enabled and the Clear sidebar widget is used.
This apparently does something weird to the HTML which we can't support (specifically the .mainContent div goes missing). This is likely something that would need to be resolved by xfrocks, the widget framework developer (sometimes he has newer versions available on his site that aren't in our resource manager).
Are you not seeing the other issues with your forum home page?
Such as when you click on the Style Chooser or a user's name? It should open content in an overlay, but that for some reason isn't working either. It's likely to do with these various JS errors that are happening.
Are you easily able to disable your ads (temporarily)?