It happens to me as well, at various times, particularly if I'm using multiquote or a reply quoting only part of a message. Usually the multiquote button sorts itself out later - but it can be a day or so later. I don't think there's any quick fix as sometimes it happens to the admins and I think even to
@Niall
I will say that I've found that usually, I can hit reply to a (whole) message and that works. When that doesn't work, it usually seems that the system is rejecting a URL or a media link in there - or less commonly an abbreviation or piece of text speak (maybe something that the autocorrect system picks up).
So the reply to your, "Is there anything you can do to help please?" is errrr, well, not exactly, usually. But maybe one day it'll stop doing it