Some users getting that error message have claimed that they were able to get the message sent by rewriting the text and removing any special characters. I do not know if the special characters were the problem, or if the error was the result of a glitch -- perhaps the messages would have gone through eventually without changes. I do not know. Or perhaps the users were running into a limit to the number of messages that could be sent in a certain period of time.
Rewriting the message and sending again might be worth trying.
When other methods fail, going to the user's feedback or profile page and using the contact link there to compose a new message seems to work fairly reliably.
