difficulty replying to comments

I'm having difficulty replying to the comment in my post https://lemmy.sdf.org/post/745908 .

This was my first attempt at replying to a comment (I just had my first success creating a top-level comment on a post -- not sure if/how that's different)

Not sure if the problem I'm running into is because of my web-browser or system (Linux desktop), or because of something about the nature of the comment I'm trying to reply to (appears to originate in Mastodon-land).

Any ideas? Maybe someone feels like trying to respond to the comment and give me a "Works For Me" response?

Edit: It seems to work just fine in this post. Still interested if someone other than me could try replying in the other thread/post.