It's not a Windows 7 problem. It works perfectly fine on my new Windows 7 laptop at work. But for some reason it doesn't work here. Must be something peculiar with my PC. And Diana's.
The submit is not acknowledged, but the post is accepted and turns up when refreshing the page.
An identical case has been reported by JP Bill in http://forums.cnet.com/7723-7598_102-556739/submit-not-acknowledged/
But with him it disappeared for some reason.
I'll see if I can do some experiments to find the cause somewhere this weekend. My guess is an add-on or a setting or something blocked by the hosts file. Don't have your engineers put too much time in it. I'm rather sure they can't reproduce it, because I can't reproduce it at work either (well, I didn't really try, but I don't have the faintest idea how to do it).
I'll keep you informed if I find something.