I am having trouble loading it at the moment as the site is timing out waiting for data from Woopra.
And you’re right. Ugly permalinks have to be turned on.
The important thing here is what has changed on the site since it was working and then stopped. And whatever that is must be causing the permalink to be thrown out during processing. Ans the fact that it was working makes me hesitate to start discussing things like mod_security as that would almost certainly have caused problems – were it going to – from day 1.
It might be worth visiting the forum admin > Integration > Page and permalink panel and updating the forum permalinks but this shouldn’t really resolve this as if they were incorrect I would expect to see 404 pages. and, in fact, I am seeing the correct urls in all of the forum links so the correct requests are being made. But give it a try.
On a side but related note – when I was looking earlier I did notice that your jQuery libraries are being loaded from the Google CDN which will cause issues (not redirection though) with Sp and many other plugins. This codex article explains the problem and what to look for:
http://codex.simple-press.com/codex/faq/troubleshooting/what-is-this-jquery-conflict/
If you are lucky you may be able to turn this off in your theme options.