@gnomebe,

This is a result of copy and pasting some content with HTML time.  Next time use the [T] button to paste copied content. We cannot help you this issue because we cannot go through all tags of your post contents. Try to delete this topic and create the same topics again. Also make sure the signature of the users doesn't contain unclosed tags.

Or, you can install phpMyAdmin plugin, brows wp_wpforo_posts table, find the body of these posts and edit them. Don't forget deleting all caches in Dashboard > Forums > Dashboard admin page after each change to see it on the front-end.

BTW, don't check unclosed tags using browser console, it automatically close tags without any logic and crashes the layout, so you'll not find any unclosed tags in browser debugging tools.