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.

Okay, it appears to be a very specific failure--I was able to respond to other comments in the thread but not the one posted by KD.

Now I'm thinking it's probably a Mastodon-to-Lemmy thing. KD's comment (appears to have) came from Mastodon.

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.

Let's do a test