Yeah, I believe that happens if your session times out. I will be installing the latest website on Saterday with all sorts of fixes and security updates though. I don't think this is a bug, but rather a security feature to prevent someone from hijacking your session if you are taking a long time to post. If you take too long the session will be restarted and a new session ID (code) will be generated, thus giving you that error, because the two codes do not match anymore.
I'm also going to enable SEO again, but I'm going to test it first tomorrow on the offline site on my PC, so that I don't break the forum or download section again.