When I do the same as you, the message “The message was successfully sent to …….xxxxxxx” appears across the top of the screen straight after the message is sent. That is how it is programmed to work. So I can’t explain why you get that empty screen. Perhaps just a server delay? Try waiting for the screen to refresh after sending the message (a guess).
Alternatively, see if there is any error message corresponding to that exact time in your server error log. I can’t imagine what error that might be, but don’t have anything else to offer I’m afraid.