Do you have a space navigator by 3D Connexion?
I had the same situation when I upgraded to WF4. You can try to update your drivers for it,
but if that does not work, you will have to disable/uninstall the device.
I contacted 3D connexion concerning this, and they say it's PTC's fault.
I am using WF 4.0 build 060
About a month ago we started using the SpacePilot Pro from 3DConnexion and I noticed the undo button was always gray.
I just took Mark Nelsons advice and sure enough It works! Thanks Mark, you deserve a raise!