I can only guess that maybe you had timed out from the server. If it happens again, try copying your response to the clipboard, closing your browser and re-opening it. That will ensure you get properly logged in again, then you can paste from the clipboard to a new message and try again.
Alternatively, if you're using a desktop computer, open the community in a new tab or window. In that window, explicitly log out and back in again. Provided you're not in a private window, the new login should apply even in your old window, so you can retry the operation which was failing.