Am currently unable to add, update, or delete plugins, and am unable to add or remove users. I did not take any actions to prompt this change to my knowledge.
I am able to publish content to the site normally. I can add, edit, and delete posts, pages, and custom posts.
So far, in my attempts to correct the admin problem, I have deleted and uploaded a fresh version of my wp-admin directory, in case something corrupted over there was causing the problem. That had no effect.
Has anyone else had this problem on 3.5.2? What did you do to fix it?