Sorry about that.
The reality is that it's impossible to provide specific diagnosis without particulars (which site, what are you doing exactly, etc.); I would suggest contacting the admins of the site in question.
What jumps out at me immediately in the link I provided, that could potentially be helpful, is:
that Wikipedia article wrote:
The replacement character ... often a black diamond with a white question mark ... is used to indicate problems when a system is not able to render a stream of data to a correct symbol. It is most commonly seen when a font does not contain a character
It goes on to say that such situations might arise because of data incompatibilities.
One thing to check is, does your text contain any "special" symbols, such as accents, smilies, Arabic, etc.? If so, then that could have something to do with it. If not, then perhaps there is a bug on the site. Between sentences is a toughie for me. If it were at the end of lines, that would be more suggestive to me.
Either way, I'd contact the admins directly. Or, open a thread on the site itself about it, because other users using that site
may have experienced the same problem.
Sorry, that's all I've got.
Maybe someone else has more...