Well, I tried this with Chrome and I got more info than IE8 or FF3, at least more of the page rendered. When viewing the source in Chrome the error message points to this:

HTML Code:
<div class="centerColumn" id="wordpressDefault"> 
<rss version="2.0"
The xml declaration is not there, it should be between the div and the <rss version. You wouldn't happen to have php short tags enabled in your php.ini would you?

It's hard to try to diagnose what is going on remotely, there are a number of possibilities why this could be happening. Did this ever work?

I think there is still some redirection problem going on. when I type in /knowledgebase/?feed=comments-rss2 it takes me to /knowledgebase/feed/. Have you installed any url/seo affecting modules in wordpress? I wounder if your zen cart seo url module is causing an issue?