Mac App Store completely broken in Mavericks??

Ever since I updated to Mavericks, the App Store has been completely useless for me. I had 6 pending updates, but whenever I click on "Update" (whether from the Updates page, Purchases page, or the app's page itself), I put in my Apple ID, and then the "Update" button goes white for about 2 or 3 minutes, then I get an error popup saying "An error has occurred", and the same thing in little red text next to the Update button.

I have done absolutely everything I can think of - I've signed out and back in to the App Store, restarted the computer, even repaired my disk permissions. Now to top it all off, one of my updates (iMovie, which I was very excited about updating), has now disappeared, and now says "$14.99" as if I never bought it, although strangely all my other updates including iPhoto are still listed there. What the **** is going on???