One forum has changed http to https but that broke mark all messages as read button. Sometimes it works and sometimes not. And while reading the thread so still it looks like it's unread as well on the front page. How to fix that? I don't have skill to do that.
This may be of help to you: repair settings (http://download.simplemachines.org/index.php?thanks;filename=repair_settings.php). Understanding where it goes and what it does: What is repair_settings.php? (http://wiki.simplemachines.org/smf/Repair_settings.php)
Another useful topic: Converting to https, step-by-step... (https://www.simplemachines.org/community/index.php?topic=555034.0)