Our community found a possible bug, that I can not see documented in here:

When you login into the forum and you click on a subforum that has more than one topic with unread comments, and then you click on the top topic (the latest) that is marked as unread. And then you go back to the overview of all the subforums - then the icon for that particular sub forum is marked as read although you got unread posts in that subforum. This only occurs if you click on the latest unread topic and go back to the overview.

Thanks for a great forum!

/Tony