The trouble don't come for my graphics card indeed,
because the system has already worked fine.

I probably have mistaken a setting without being aware of it,
and the error came out after the xserver has reset.

I suppose something is mismatching between metacity and compiz.
Compiz is needed for the visual effects can work. But I tried to uninstall
all its components, then to rename the folders /home/myId/.compiz and
/home/myId/.config/compiz and after to re-install all the compiz components.
Nothing better, at each boot or session opening the visual effects have turn
to none (and till I don't change to normal, my windows decoration has
disappeared,
with the 3 buttons too - close , enlarge full screen, reduce to tray - )