This occurs because of incompatible characters. The characters in question are quotation marks (left and right), apostrophes, and emdashes. If I manually replace them with ", ' and -- I don't get the error message. When I do get the error message, I have to abort that message and start a new one, as fixing the existing message and resubmitting gives a message that I already posted.
SMF 2.0.15 with Postgres. (I don't operate the server so am unsure how to get the Postgres version #, but could find out tomorrow during business hours).
Is there a patch or mod I need to fix this issue? Or is there a setting I need to tweak?