strange problem with iTrader and vBSEO URL rewriting

I was customizing the iTrader 2.0.1 mod for my plantswap.net forum today and noticed something strange. When viewing the forum (iTrader page) as a guest, some of the URLs are not rewritten while others are. For example visit this page:

Plant Swap - iTrader - needmoresoil

The third rating links to a vbSEO re-written URL, but most of the rest do not. If you click on any of the non-re-written URLs, you will get an error message instead of the correct thread.

If you view the same iTrader page while logged in, all URLs are rewritten as expected.

Any ideas how this could be happening (or suggestions on how I might troubleshoot it)?

Hello Bernard ,

Plant Swap - iTrader - needmoresoil url gives me 403 warning