Sometimes when you open a Post Reply screen, the normal icons do not appear. And if this happens, there is no point in trying to make the reply because nothing will happen when you try to post it. This is different to the problem where the spinning cogs go on for ever - not even these will appear.

I have found that if you find that the normal icons don't appear, before you start the reply, close the Post Reply screen, then click on the Permalink icon which appears at the bottom left of every post apart from the opening post. Now reopen the Post Reply screen and I think you will see that the icons are now there and you can post your reply.

The Permalink icon looks like

I haven't been able to check this out exhaustively, but it is worth trying this out.

As an alternative to clicking the Permalink icon, refreshing the page generally also works.