BIG-IP : redirect loop : debugging techniques

I completely agree with John here. There's absolutely nothing in your iRule that could be generating a redirect. I'd be willing to bet that if you did a tcpdump between the F5 and the web server, you'd see this 302 message coming from the server itself. I know you're generalizing the code for forum view, but is there anything different between accessing directly and through the VIP, other than the host name? If not, try adding this to your code:

HTTP::header replace Host "my-real-server-hostname"    

where "my-real-server-hostname" is the name you use when you're accessing the web server directly.