First off, the validatation has nothing to do with your particular problem. If the page had completely invalid code, it would not even display anything. As it is, it is valid if not "strict" hand-coded HTML that displays correctly across all popular browsers - even the Sony PSP's dinky browser works perfectly. When I bothered looking at validators almost all of the errors were conformaty annoyances - such as adding ALT text on every single image (even ones that shouldn't have any alt text because they aren't functional). So, as long as the page looks exactly as intended in a wide range of browsers, I'm happy.

When you make a post you send information to the server, then the server sends back only one bit of text, a Location: redirection to either the forum index or thread, depending on your preference. Then the browser simply goes to that page.

I tried turning off the automatic redirects in Opera and I saw the redirect notice, properly formatted, as intended. Otherwise, with redirects on, it works perfectly for me 100% of the time... and I can't troubleshoot a problem that I can't see myself... :-(