He’s running into the limit set by your max consecutive replies site setting. That setting defaults to 3 and is applied to all non-staff users on the site.
Should not the text state that though and not “Maximum number of replies a new user can make…”?
Also as I have users that write blog entries on their topics and assuming they don’t have anyone replying to them, is there a time element involved in this check? Meaning is it more than 3 consecutive responses in a single 24hr period or what?