So I received an email which says there is a new topic published in the forum. But when I click on the link I received a 404 error. And there is a notification in the moderation menu which indicates there is an unapproved topic waiting moderation, but when I click on it there is nothing to approve.
I tried cleaning the cache but no luck. I'm pretty sure that topic exists.
Make sure that you are logged in as an admin. When you click a topic link, it opens a forum where you are not authorized, so it brings a 404 error. Make sure you use the same browser. The mobile email applications use their own web browsers where you are not logged in.
Sometimes spammers delete their topics right after publishing it, so this also could be the case.