Bump. I have the same issue.
Looks like it was a plugin incompatibility. Not sure which one, because I had a few that got upgraded today (it was between Advertising Manager, All in one SEO, or Intense Debate).
Next time I have weirdness during an upgrade I’ll be sure to turn off plugins if I’m having issues.
My widgets are still there, but I can’t “open” them up and change their settings. After reading this I’m afraid to add any new widgets to test the situation.
