It is possible to post a comment in a category (instead of a forum) using add-topic/<forum-number>. Which it should not be. Please try this URL https://wpforo.com/community/add-topic/6/ for example.
Please note you can not post a comment in a category. You can choose only the forum, not the Category. See the recorded video below. https://www.screencast.com/t/uHtjn7um6KTA
Also please read the following changelog: https://wpforo.com/community/wpforo-announcements/wpforo-1-8-0-is-released/