And finally a deadly Xid. That is, once this happened, the whole screen is locked and only the mouse can be moved, all the rest is dead. I managed to restore the system by logging in from the network then killing X11. Here's the Xid:
[146849.637279] NVRM: Xid (0001:00): 6, PE0000 0374 00000000 0000fdcc ff3b5998 00000001
[146849.654601] NVRM: Xid (0001:00): 30, L1 -> L0
That's it. Hopefully that can help the nvidia engineers. I'm available if you need some more testing.