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Yesterday Blender was working fine. Today it isn't recording any history, I can't undo anything. Also, when I add a new mesh, all the options in the Operator section of the Tool Shelf are grayed out, I can't do anything with them.
I checked User Preferences, it shows 32 steps in the Undo section of the Editing tab. I tried undoing by the Ctrl+Z shortcut, pressing the "Undo" button in the Tool Shelf, and clicking on the "Undo history" button, which does nothing. I tried closing Blender and opening it again. Rebooting does nothing. It was working fine yesterday. I have 2.76 running on Ubuntu 14.04 64-bit.
I need to go ask elsewhere how to check the most recent changes to my system - the system's software updater downloaded 40-odd MB of stuff this morning, but I don't know what it was.
Should this be reported as a bug? Should I uninstall and reinstall Blender? When I open it, the mini-splash screen has a date and time in the top right corner - I'm guessing that indicates the last time the program was changed by an update. That date is a week ago, so that isn't it. Does that definitively mean the problem is with my OS?