Nvidia prime black screen on boot

[giacof]

$ lsb_release -rd
Description: Ubuntu 15.04
Release: 15.04

$ apt-cache policy nvidia-346 nvidia-prime
nvidia-346:
  Installato: 346.59-0ubuntu1
  Candidato: 346.59-0ubuntu1
  Tabella versione:
 *** 346.59-0ubuntu1 0
        500 http://it.archive.ubuntu.com/ubuntu/ vivid/restricted amd64 Packages
        100 /var/lib/dpkg/status
     346.46-0ubuntu1 0
        500 http://developer.download.nvidia.com/compute/cuda/repos/ubuntu1410/x86_64/ Packages
nvidia-prime:
  Installato: 0.8.1
  Candidato: 0.8.1
  Tabella versione:
 *** 0.8.1 0
        500 http://it.archive.ubuntu.com/ubuntu/ vivid/main amd64 Packages
        100 /var/lib/dpkg/status

$ lspci -v
[...]

01:00.0 VGA compatible controller: NVIDIA Corporation GK107GLM [Quadro K1100M] (rev a1) (prog-if 00 [VGA controller])
        Subsystem: Dell Device 15cc
        Flags: bus master, fast devsel, latency 0, IRQ 38
        Memory at f3000000 (32-bit, non-prefetchable) [size=16M]
        Memory at d0000000 (64-bit, prefetchable) [size=256M]
        Memory at e0000000 (64-bit, prefetchable) [size=32M]
        I/O ports at e000 [size=128]
        [virtual] Expansion ROM at f4000000 [disabled] [size=512K]
        Capabilities: <access denied>
        Kernel driver in use: nvidia

I'm using Nvidia prime with a Quadro K1100M card.
When I switch from intel to nvidia, then after booting I randomly get a black screen [about 50% of the times]. EDIT: after latest kernel update, now it happens ALL the times!

I can't but switch to a virtual console by pressing [Ctrl+Alt+Fn] and then reboot.

output of nvidia-bug-report.sh attached

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.