Did you figure this out? Having exactly the same problem, v3.3.1
Yea, I got it figured out. Apparently, there are a few different reasons for this happening. In my case, it was a pretty easy fix. My host (bluehost) had changed their PHP config from 5.2 to 5.4. I’m not really versed on this, but that was the only thing that had changed. So, I just reverted it back to 5.2, updated everything on WP, then went back to 5.4. After that, it was fine. From what I’ve read though, its usually the add-ons on you wp-admin. They say to disable those first, then one by one go through them to see what the problem is. Hopefully that helps. good luck