Reply To: how to create dynamic reply-box
Ok. I found the problem: Buddypress.
The functionality of reply.js works for bbpress, but does not work if the topic is in a forum that is within a Buddypress group.
Just tested on your site, as well as my test site. And yes that BuddyPress is related to this issue.
May not specifically be BuddyPress and their code. I think there are tickets with bbPress’ BuddyPress extend code that might have issues with some conditionals in a bbPress post that is in a group.
Will look through bbPress trac for some possibly related tickets.
Edit: Found the ticket.
https://bbpress.trac.wordpress.org/ticket/2974
It is pretty easy to workaround this issue as well, since all you have to do is enqueue the javascript with a different conditional or enqueue it sitewide, but doing it sitewide might cause another issue somewhere.
Will do some testing and get back.