Cut your prefs folder, restart
For whatever reason (I would really like to know why) your prefs folder is probably the problem.
Go into your Documents\maya\pick your maya version folder
Cut your prefs folder, paste it somewhere else (this is to ensure you have a backup) and restart Maya.
Maya will re-create your prefs folder from default and you should be able to access the Expression Editor.
I am going to try to narrow down what file is causing the issue, and report back what I find.
Update: It was the userPrefs.mel file in prefs that was causing the issue for me; I wish I could tell which part of it was causing the bug, but alas, no luck. Anyone have an idea as to why this happens sometimes?
Last edited by jengy; 02-01-2014 at 09:23 PM.