It appears that the problem is related to switch the virtual
terminal when it is in the vt7 and change to another one, like vt8
(it is made automatically in the xubuntu new screen saver). You can
experiment the same results switching to another VT (Ctrl+Alt+FX)
and switching back to the vt7.
I installed xscreensaver that do not change the VT when it is
activated and the problema was "solved" (ok, it's not solved, but
just not activated)