Pilot6, нашел решение на
http://askubuntu.com/.
If I close the working window, and reopen, it no longer works.
No clue as to cause, but my quick, non-destructive workaround is to switch to a VT and back: For example,
Alt + Ctrl + F5, then when you get the login prompt, simply press
Alt + F7 to switch back to the GUI, and everything is back to normal, with all windows still open.
Правда это нужно делать при каждой загрузке убунты, но хотя бы во время работы все хорошоэ