PaulTiger you are correct - as of now, you can only post a forum message by starting a New Message - the Reply function has no Submit button anymore.
Agoracom did a software "update" tonight, and maybe Microsoft "helped" them. I have zero Microsoft stuff on my machine, so I don't get those "enhancements" (just teasing. I also have no virus protection and spend a total of zero minutes a year dealing with that).
I have a hunch it will be a priority item for Agoracom to fix - everyone will notice it.