There’s another thread on this. In my case it was a database permission issue. I opened up the WordPress MySQL user to full-on everything on the WordPress DB, restarted Apache and refreshed MySQL just to be sure, then it resolved itself. Post-update I put my previous permissions back into place. There are others saying that this didn’t work for them, so take it with a grain of salt.
Check your database permissions; this was an issue for me. During the upgrade I gave the WordPress user full permissions to the WordPress database and it resolved the issue. I then peeled the permissions back to their previous state.
