I had the exact same issue and followed advice from @pfaffman on this thread. All you need to do is logon to the server as root and do a pull and rebuild of the app and it will all be fixed:
cd /var/discourse
git pull; ./launcher rebuild app
Of course, the bigger question is why a few of us are having this exact same issue (including the hamburger not working and then the admin page giving a does not exist error) during upgrades when it has never happened before?