The link you posted to works fine. The thread isn’t closed, so you should be able to reply.
If threads aren’t allowing you to reply sometimes, all you would need to do is clear your Browser Cache. You can do this easily by clicking your RELOAD Button on your browser.
Check out this link where they mentioned this for more info – https://appfinite.com/topic/having-trouble-logging-in/ (it’s a common issue with the bbPress plugin that a lot of sites have been dealing with)