I did not expect turning off the cache plugin to resolve the problem but it gets in the way as reloading a page from the cache will display the old page that may now be fixed.
basically the page stops loading at that point so there is no footer which loads a lot of the code – including thing like the scripts needed for the add reply.
My initial thought is that there is something wrong with the spTopicView.php theme template. I would have expected to see a php error message but you may have those suppressed.
You made me an admin account but not an SP Admin. So – can you please go the the forum admin > toolbox > error log and see if there are entries for today. Ignore older ones. If there are copy and send me the last 2 or 3.
Meanwhile I would suggest you reload the Sp theme files – in particular the spTopicView.php file which I suspect might be corrupted in some way.