In your original log, plymouthd hit the same Nvidia kernel bug:
Aug 11 22:41:26 voldemort kernel: watchdog: BUG: soft lockup - CPU#6 stuck for 26s! [plymouthd:322]
So I think there are two problems here:
1. A buggy Nvidia kernel driver that keeps crashing (at least on this system); and
2. An incomplete Nvidia driver install that's trying to use the wrong DDX driver for the Nvidia GPU.
In both cases, please uninstall the nvidia driver and try a different (older) one. You can do so using the 'Additional Drivers' app.
In your original log, plymouthd hit the same Nvidia kernel bug:
Aug 11 22:41:26 voldemort kernel: watchdog: BUG: soft lockup - CPU#6 stuck for 26s! [plymouthd:322]
So I think there are two problems here:
1. A buggy Nvidia kernel driver that keeps crashing (at least on this system); and
2. An incomplete Nvidia driver install that's trying to use the wrong DDX driver for the Nvidia GPU.
In both cases, please uninstall the nvidia driver and try a different (older) one. You can do so using the 'Additional Drivers' app.