I realize I should upgrade, it's just a long procedure each time so I put it off until it's necessary.
The error I get in my 404 log is:
If I look in the source of that page I see:
var baseurl = "/forum";
var open_block = new Image();
open_block.src = baseurl + "/images/general/default/toggle_open.gif";
var closed_block = new Image();
closed_block.src = baseurl + "/images/general/default/toggle_closed.gif";
So as you can see the URL should have /forum/ in it - but it does not.
I did look through the upgrade logs and didn't see this addressed - did I miss it?