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.
I've been running into a similar issue. I click Post and get the activity spinner on the button. Then nothing. I've had this occur on three or four threads. Closing the tab and opening a new instance / reloading the page in the existing tab does not solve the issue. It appears to be local to specific posts / threads. I haven't tested to see if I can reply to a different comment on the affected threads.
I ended up sending a DM to someone because I couldn't reply in the original post. I linked to it. I just confirmed that I still couldn't reply to that post. Here's a screenshot.
I've been having exactly this problem on both the web client and from Memmy, an iOS client, while trying to reply to this local post. It was posted by someone from a Mastodon domain, so maybe related to @bsammon's idea.