The quick reply box has had some problems lately. You can try two different things when posting a reply on an existing thread.On my Galaxy S10+ I can't post any replies. When I try to type into the quick reply box nothing happens. I am logged in and it allows me to start a new thread.
The other option is using the Android Central Forums app. In general I prefer this app over the main AC app that you are using, more options and a cleaner experience.This is working if I select the reply option when tapping a message. I tried the other option with typing in quick reply and then tapping it again. No matter how many times I try it won't allow me to type in the quick reply box.