Unable to respond/reply to a group text - solution

I've had this same issue multiple times on my iPhone (that has all the latest updates). I tried all the suggestions of closing the app, rebooting the phone, etc, and nothing worked. Finally I figured out how to fix it: Settings, Messages, turn iMessage off (and then back on if you want, though it can happen again).

Note -- I tried to reply on multiple other threads which had this same issue, but I was unable to respond since they were old threads, so I figured I'd just provide this comment so that other people in the future can find the simple solution.