Please report here if you get any e-mails notifying you about a "New reply" and when you click on the link, it gives you an error page.
It will be something like this:
Not Found
The requested URL /forum/index.phpshowtopic=48380 was not found on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
Based on the browser you use it could look a little different.
Thanks