Re: 404 errors on all posts – please help
I would say this is a mod_rewrite and permalink issue. If it was working before, then something changed with the host, maybe the .htaccess file in the forum directory, or the parent one. But, all you need to do to get around it, for now, is turn off the permalinks, to false.
This URL works fine for a second:
Then it redirects to a permalink URL:
Maybe you moved the forum recently and did not update the permalink structure?