The funny thing is, ad-blocking add-ons for Chrome were blocking any and all script trying to run in that thread, like, edit, add reply, even the 3 share buttons weren't showing. Because the title contained the word "Advertising"
Before you think its due to custom filters, this happened to 3 different users, who aren't using the same settings/filters. This is what Chrome Dev tools showed when add reply button was being clicked (other buttons had the same result appearing)
Since this isn't a issue with XenForo, I'm creating this in the forum management thread so that other forum admins could be aware of this, and if users report not being able to post on some thread properly and that you "add reply" button does nothing, well, you know whats the issue! Not sure if this is an issue with only one ad-blocking software or multiple, or even if its cross browser, but it certain is a problem with Chrome + AdBlockerPlus/AdBlocker.
Let's see if this issue repeats itself here on this thread on XenForo, if you can't reply when you click "add reply" and have add-block enabled on XenForo, disable it and try to post.