The freeze is only related to X, I guess. If, after triggering it, I'm quick
enough to switch to a tty then the system is still responsive, I can log in and
input commands to the shell (this is how I got the dmesg after the freeze). If
instead I wait a bit after the freeze then the keyboard and the mouse become
unresponsive, and pressing Ctrl+Alt+F2 has no effect. The system gets stuck
with a black screen, the mouse cursor can still be moved around but clicks have
no effect. The same happens when switching back to X from the tty after the
"quick switch to tty" mentioned above.