OK. I reinstalled the "nouveau" driver and reproduced the bug. I was obliged to reboot the system because everything is frozen except the mouse cursor: no access to any terminal.
Then I typed in a terminal: $ sudo apt-get install xdiagnose $ sudo apt-get install python-apport $ apport-collect -p xorg 1453812 ERROR: Could not import module, is a package upgrade in progress? Error: No module named PyQt5.QtCore -- You received this bug notification because you are a member of Desktop Packages, which is subscribed to xorg in Ubuntu. https://bugs.launchpad.net/bugs/1453812 Title: System freeze when using LibreOffice Writer Status in xorg package in Ubuntu: Incomplete Bug description: LibreOffice Writer is freezing the system after a few minutes (Ubuntu 15.04, libreoffice-writer 4.4.2-0ubuntu1): the only possible action is moving the mouse pointer on the screen. Impossible to connect to a tty session to kill Xorg. A physical reboot is necessary. Note that I used LibreOffice Calc without problem. The problem has been encountered also under Fedora 21: http://forums.fedora-fr.org/viewtopic.php?id=63428 The solution proposed on this french forum is to replace the "nouveau" driver by the "nvidia" proprietary driver (340.76). It works for me. To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/xorg/+bug/1453812/+subscriptions -- Mailing list: https://launchpad.net/~desktop-packages Post to : desktop-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~desktop-packages More help : https://help.launchpad.net/ListHelp