Seeing a lot of this since yesterday:

The requested URL /forum/images/ajax_preview.png was not found on this server.

This image is trying to load on every line of the forum display and is failing.

So that image (at a different URL) is the little cartoon bubble at the end of every line that lets you see the content without actually opening the post. The problem is, it's not at /forum/images/ajax_preview.png, it's at (usually) /forum/cache/[some numerical version]/themes/[your theme here]/images/ajax_preview.png. If your browser has lost track of where it's supposed to be, this is going to be a problem. I'd clear my cache and see if that fixed the problem.

(I just checked to see if somehow that image had gotten deleted from a theme directory - all current themes work.)