Reply To: replying to topic issues
I had a feeling you were using that plugin, that is why you linked to the single reply post.
There is a little shortcode you have to change in the plugins settings for the notification. It should have it this way by default, but it does not. Might need to contact the plugin author to set that as the default setup.
I think you have to change [reply-url] with [reply-replyurl] in the plugins settings. It adds the settings to bbPress default settings in Settings > Forums.