I've found that if the message body is blank the form will fail on submit. Just fixed mine by simply adding a message body of course but It may be a topic of confusion for others because the error message doesn't mention this. Typically the error message received is "Failed to send your message. Please try later or contact the administrator by another method." A recommendation for improvement might be to add a space by default to the message body if it's cleared out or prompt a more appropriate error message if possible.