I have already fixed the error. I think the problem was caused when I installed the Gym Sitemaps. But because I was not able to make it work, I uninstalled it and I think this triggered the problem. It's a good thing I have a demo forum where I first installed the phpBB SEO premod. What I did to fix the problem is just to copy the whole phpbb_seo folder from the demo forum and pasted it to the live forum and now if I edit a post and submitted or saved it, the 404 error does not appear anymore. The only problem is that it is redirected to a different URL which is the original URL without the SEO rewrite. Please see below URL examples
This is the URL for my post. http://www.itchytech.com/forums/microsoft-will-halt-service-pack-blockers-for-vista-and-xp-t6.html
If i edit this post and saved it, it is being redirected to this URL http://www.itchytech.com/forums/post6.html#p6
Any Idea on how it will redirect to the URL withe SEO rewrite?