I get a lot of broken posts like this
Looks like posted HTML doesn't get sanitized
Your content is crashed. You've inserted incorrect content and crashed the HTML. You should edit the content and try to fix HTML code of your posts. In other words this is a result of inaccurate data inserting, there is no any bug or problem from wpForo side, the problem comes from your crashed HTML code in the first post of this topic.
Please read my answers in the topic above and be careful when adding a copied content in the post editor.
BTW, when you're adding YouTube Video URL in the post, don't put it as link, make sure it's plain text URL, so just edit the first post and remove the link from YouTube URLs, just insert a URL.
Maybe it isn't dangerous, but it breaks the page and it's annoying.
There's no way to control what users post, and there's no way to edit the post so ... what's the solution ?