I also get that error statement at the bottom of all threads.
I noticed this forum format is very different than most in that way.
I figured out to use that button at the TOP of the page to reply. I also figured out that to jump to the next page in the thread list, or within a thread, I have to use the page link at the TOP. And there is no <Next> button so I have to click on the page number. As long as I know what works then I can do what needs to be done.
edit: looking again I guess there are page number links at the bottom of the thread list but just not within a thread due to that error message.