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.