Weird, haven't seen that before. I can't imagine it'd be an issue with responses, the only difference between a response and a new message is the content. There's a ton of PMs sent every day too so if it was widespread there'd be more people talking about it I'd guess.
So could be something weird with the content itself (i.e. special characters)
You could try replying to the PM, but change the recipient to me instead of the user (I won't keep it if I get it I'll delete it to keep it private), and see if it still gives an issue or not.
Uncertainty is an uncomfortable position.
But certainty is an absurd one.