Oddly, I’ve done that more times than I have “odd unread answers that I cannot read”. There are only two of the latter for me - the BRT answer 6 (being discussed here) and one from a global moderator about commenting being turned on in the first place.
It just looks like a bug - replying to an answer by “reply” is blocked, but replying to an answer by “quote” is not.
Edit: Added link to the other topic that shows the bug for me