Kris (@awesomerobot) others on the site have been experiencing the same problem, clearing caches etc. had no effect and the problem is across a range of browsers. The problem started this morning while I was typing a long post (again!?) and in the middle of writing the post I attempted to check on another post and momentarily got a “not available” screen! I returned to my in progress post and completed it succssfully and posted it.
However, I then spotted some spelling and grammar errors and when I went to edit the post I found the tiny box to enter the text!
One user also reported “I’ve noticed that visiting a topic and then going back to the root level of the forums doesn’t always mark the topic as read. That seems to have started this morning when text box size issue started”
At a guess the site started behaving in the way that it is after an update.