I didn't change anything with the subject of my reply post, but as seen I got that error. Even when I only preview the full quote without writing a reply below it.
This a known bug here, some user reported it too in the Bugs & Tech support board on this
thread. I even experienced it sometimes even when clicking the preview button, some users' experience is when they click the post button.
What you need to do is refresh the page and try again, better to copy the text you have written before doing that to save you some time.