This may have been due to two things. 1, the .htaccess file itself had some unusual code in it; I've renamed it to .htaccess.bak to help rule it out. Arguably more important is 2, which is related to 1; the .htaccess was causing /forums to rewrite to forums.php which didn't exist, hence the 404. The other concern is that it isn't /forums, it's /forum (your hyperlink in the original post is broken). In either case it looks to be fixed now, but you may want to check the code in .htaccess.bak to see if any of the code there was necessary.
