"Control+Alt+<some key>" shortcuts, as well special keys (such as sound management keys) stop working randomly

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http://askubuntu.com – I am using Unity 2D in Ubuntu 12.04 in a Lenovo ThinkPad T420. Sometimes, Unity 2D crashes. It restarts by itself; however, when it happens, shortcuts in the form Control+Alt+[some other key] just stop working, both the default ones (such as Control+Alt+L for locking screen) and the ones I have created myself. Also, the keys for managing sound volume and to mute sound stop working, too. The only way to make them work is to end the session or to restart the computer. How could I solve it? I can live with the buggy Unity 2D but I need the shortcut keys. Does at least someone have the same prob (HowTos)